Benzo[a]Anthracene Compound, Light-Emitting Element, Display Device, Electronic Device, and Lighting Device

ABSTRACT

Provided is a light-emitting element with high emission efficiency including a fluorescent material as a light-emitting substance. In a light-emitting element including a pair of electrodes and an EL layer between the pair of electrodes, a delayed fluorescence component due to triplet-triplet annihilation accounts for 20% or more of light emitted from the EL layer, and the light has at least one emission spectrum peak in the blue wavelength range. The EL layer includes an organic compound in which an energy difference between the lowest singlet excited energy level and the lowest triplet excited energy level is 0.5 eV or more. The EL layer includes a benzo[a]anthracene compound.

This application is a continuation of copending U.S. application Ser.No. 17/112,274, filed on Dec. 4, 2020 which is a continuation of U.S.application Ser. No. 16/356,062, filed on Mar. 18, 2019 (now U.S. Pat.No. 10,862,041 issued Dec. 8, 2020) which is a continuation of U.S.application Ser. No. 14/926,861, filed on Oct. 29, 2015 (now U.S. Pat.No. 10,236,448 issued Mar. 19, 2019), which are all incorporated hereinby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a benzo[a]anthracenecompound. One embodiment of the present invention relates to alight-emitting element in which a light-emitting layer capable ofproviding light emission by application of an electric field is providedbetween a pair of electrodes, and also relates to a display device, anelectronic device, and a lighting device including the light-emittingelement.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method of driving any of them,and a method of manufacturing any of them.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting material (an EL layer) is interposed between a pair ofelectrodes. By application of a voltage between the electrodes of thiselement, light emission from the light-emitting material can beobtained.

Since the above light-emitting element is a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, and low power consumption.Furthermore, such a light-emitting element also has advantages in thatthe element can be manufactured to be thin and lightweight, and has highresponse speed.

In the case of a light-emitting element (e.g., an organic EL element)whose EL layer contains an organic material as a light-emitting materialand is provided between a pair of electrodes, application of a voltagebetween the pair of electrodes causes injection of electrons from acathode and holes from an anode into the EL layer having alight-emitting property and thus a current flows. By recombination ofthe injected electrons and holes, the light-emitting organic material isbrought into an excited state to provide light emission.

Note that an excited state formed by an organic material can be asinglet excited state (S*) or a triplet excited state (T*). Lightemission from the singlet-excited state is referred to as fluorescence,and light emission from the triplet excited state is referred to asphosphorescence. The formation ratio of S* to T* in the light-emittingelement is statistically considered to be 1:3. In other words, alight-emitting element including a phosphorescent material has higheremission efficiency than a light-emitting element containing afluorescent material. Therefore, light-emitting elements includingphosphorescent materials capable of converting a triplet excited stateinto light emission has been actively developed in recent years.

Among light-emitting elements including phosphorescent materials, alight-emitting element that emits blue light in particular has yet beenput into practical use because it is difficult to develop a stablematerial having a high triplet excited energy level. For this reason,the development of a more stable fluorescent material for alight-emitting element that emits blue light has been conducted and atechnique for increasing the emission efficiency of such alight-emitting element has been searched.

As an emission mechanism capable of converting part of a triplet excitedstate into light emission, triplet-triplet annihilation (TTA) is known.The term TTA refers to a process in which, when two triplet excitonsapproach each other, excited energy is transferred and spin angularmomentum are exchanged to form a singlet exciton.

As compounds in which TTA occurs, anthracene compounds are known.Non-Patent Document 1 discloses that the use of an anthracene compoundas a host material in a light-emitting element that emits blue lightachieves an external quantum efficiency exceeding 10%. It also disclosesthat the proportion of the delayed fluorescence component due to TTA inthe anthracene compound is approximately 10% of emissive components ofthe light-emitting element.

Furthermore, tetracene compounds are known as compounds in which adelayed fluorescence component due to TTA accounts for a largeproportion. Non-Patent Document 2 discloses that the delayedfluorescence component due to TTA in light emitted from a tetracenecompound accounts for a larger proportion than that for an anthracenecompound.

REFERENCES Non-Patent Documents

-   Non-Patent Document 1: Tsunenori Suzuki and six others, Japanese    Journal of Applied Physics, Vol. 53, 052102 (2014)-   Non-Patent Document 2: D. Y Kondakov and three others, Journal of    Applied Physics, Vol. 106, 124510 (2009)

SUMMARY OF THE INVENTION

What is important in increasing the emission efficiency of alight-emitting element including a fluorescent material is to convertenergy of triplet excitons, which do not contribute to light emission,into energy of singlet excitons, which have a light-emitting property,with high conversion efficiency; i.e., conversion of triplet excitonenergy into singlet exciton energy by TTA is important. To achieve this,an increase in the proportion of a delayed fluorescence component due toTTA in emissive components of a light-emitting element is especiallyimportant. This is because an increased proportion of a delayedfluorescence component due to TTA means that singlet excitons having alight-emitting property are formed in a higher ratio.

Tetracene compounds, which are known as compounds in which a delayedfluorescence component due to TTA accounts for a large proportion, havelower excited energy than anthracene compounds and emit yellow light orlight with a longer wavelength than yellow light. Therefore, tetracenecompounds are difficult to use as a host material in a light-emittingelement that emits blue light. To increase the emission efficiency of alight-emitting element that emits blue light, a compound which has highexcited energy and in which a delayed fluorescence component due to TTAaccounts for a large proportion is needed.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element that has high emissionefficiency and includes a fluorescent material. Another object of oneembodiment of the present invention is to provide a light-emittingelement that emits blue light with high emission efficiency. Anotherobject of one embodiment of the present invention is to provide acompound in which a delayed fluorescence component due to TTA accountsfor a large proportion of emissive components. Another object of oneembodiment of the present invention is to provide a light-emittingelement in which a delayed fluorescence component due to TTA accountsfor a large proportion of emissive components. Another object of oneembodiment of the present invention is to provide a novel compound.Another object of one embodiment of the present invention is to providea light-emitting element including a novel compound. Another object ofone embodiment of the present invention is to provide a novellight-emitting device with high emission efficiency and low powerconsumption. Another object of one embodiment of the present inventionis to provide a novel display device.

Note that the description of the above object does not preclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

In one embodiment of the present invention, the emission efficiency of alight-emitting element at least including an EL layer is improved insuch a manner that TTA is efficiently caused in the EL layer to converttriplet excitons which do not contribute to light emission into singletexcitons and then light is emitted from the singlet excitons or light isemitted from a guest material (a fluorescent dopant) via energytransfer.

What is important in causing TTA in the EL layer efficiently is the useof, as a host material, a compound in which a delayed fluorescencecomponent due to TTA accounts for a large proportion of emissivecomponents. Particularly in a light-emitting element that emits bluelight, the use of a compound with high excited energy as a host materialis important.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. A delayed fluorescence component due to triplet-tripletannihilation accounts for 20% or more of light emitted from the ELlayer, and the light has at least one emission spectrum peak in a bluewavelength range.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. A delayed fluorescence component due to triplet-tripletannihilation accounts for 20% or more of light emitted from the ELlayer, and the light has at least one emission spectrum peak at awavelength greater than or equal to 400 nm and less than or equal to 550nm.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes an organic compound in which an energydifference between a lowest singlet excited energy level and a lowesttriplet excited energy level is 0.5 eV or more. A delayed fluorescencecomponent accounts for 20% or more of light emitted from the EL layer,and the light has at least one emission spectrum peak in a bluewavelength range.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes an organic compound in which an energydifference between a lowest singlet excited energy level and a lowesttriplet excited energy level is 0.5 eV or more. A delayed fluorescencecomponent accounts for 20% or more of light emitted from the EL layer,and the light has at least one emission spectrum peak at a wavelengthgreater than or equal to 400 nm and less than or equal to 550 nm.

In each of the above structures, a difference in equivalent energy valuebetween a peak wavelength of a fluorescence spectrum and a peakwavelength of a phosphorescence spectrum of the organic compound ispreferably 0.5 eV or more.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes a compound having a benzo[a]anthraceneskeleton, and a delayed fluorescence component accounts for 20% or moreof light emitted from the EL layer.

In each of the above structures, the EL layer preferably includes aguest material capable of emitting fluorescence.

In each of the above structures, the guest material preferably includesa pyrene skeleton.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a general formula (G1).

In the general formula (G1), A represents a substituted or unsubstitutedcarbazolyl group, R¹ to R¹⁰ each independently represent any ofhydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl grouphaving 3 to 6 carbon atoms, and a substituted or unsubstituted arylgroup having 6 to 13 carbon atoms, R¹¹ represents any of hydrogen, analkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted phenyl group, and Arrepresents an arylene group having 6 to 13 carbon atoms. The arylenegroup may include one or more substituents and the substituents may bebonded to each other to form a ring.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a general formula (G2).

In the general formula (G2), R¹ to R¹⁰ and R²¹ to R²⁸ each independentlyrepresent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atoms, R¹¹ represents anyof hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, and a substituted or unsubstitutedphenyl group, and Ar represents an arylene group having 6 to 13 carbonatoms. The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a general formula (G3).

In the general formula (G3), R¹ to R¹⁰ and R³¹ to R³⁵ each independentlyrepresent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atoms, R¹¹ represents anyof hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, and a substituted or unsubstitutedphenyl group, and Ar represents an arylene group having 6 to 13 carbonatoms. The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring.

In the benzo[a]anthracene compound having any of the above structures,Ar is preferably any of a substituted or unsubstituted phenylene groupand a substituted or unsubstituted biphenyldiyl group.

In the benzo[a]anthracene compound having any of the above structures,Ar is preferably a substituted or unsubstituted phenylene group.

In the benzo[a]anthracene compound having any of the above structures,Ar is preferably a substituted or unsubstituted m-phenylene group.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a general formula (G4).

In the general formula (G4), R¹ to R¹⁰, R²¹ to R²⁸, and R⁴¹ to R⁴⁴ eachindependently represent any of hydrogen, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, andR¹¹ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substitutedor unsubstituted phenyl group.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a general formula (G5).

In the general formula (G5), R¹ to R¹⁰, R³¹ to R³⁵, and R⁴¹ to R⁴⁴ eachindependently represent any of hydrogen, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms, andR¹¹ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substitutedor unsubstituted phenyl group.

Another embodiment of the present invention is a benzo[a]anthracenecompound represented by a structure formula (100).

Another embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes a benzo[a]anthracene compounddescribed in any of the above structures.

Another embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes a benzo[a]anthracene compounddescribed in any of the above structures, and a delayed fluorescencecomponent accounts for 20% or more of light emitted from the EL layer.

In each of the above structures, the EL layer preferably includes aguest material capable of emitting fluorescence.

In each of the above structures, the guest material preferably has afunction of exhibiting an emission spectrum peak in a blue wavelengthrange.

In each of the above structures, the guest material is preferablycapable of emitting delayed fluorescence.

In each of the above structures, the guest material preferably includesa pyrene skeleton.

Another embodiment of the present invention is a display deviceincluding the light-emitting element with any of the above structures, acolor filter, a seal, or a transistor. Another embodiment of the presentinvention is an electronic device including the display device and ahousing or a touch sensor. Another embodiment of the present inventionis a lighting device including the light-emitting element with any ofthe above-described structures and a housing or a touch sensor. Thecategory of one embodiment of the present invention includes not onlythe light-emitting device including the light-emitting element but alsoan electronic device including the light-emitting device. Thus, alight-emitting device in this specification means an image displaydevice or a light source (including a lighting device). Thelight-emitting device may be included in a module in which a connectorsuch as a flexible printed circuit (FPC) or a tape carrier package (TCP)is connected to a light-emitting device, a module in which a printedwiring board is provided on the tip of a TCP, or a module in which anintegrated circuit (IC) is directly mounted on a light-emitting elementby a chip on glass (COG) method.

According to one embodiment of the present invention, a light-emittingelement that has high emission efficiency and includes a fluorescentmaterial can be provided. According to one embodiment of the presentinvention, a light-emitting element that emits blue light with highemission efficiency can be provided. According to one embodiment of thepresent invention, a compound in which a delayed fluorescence componentdue to TTA accounts for a large proportion of emissive components can beprovided. According to one embodiment of the present invention, alight-emitting element in which a delayed fluorescence component due toTTA accounts for a large proportion of emissive components can beprovided. According to one embodiment of the present invention, a novelcompound can be provided. According to one embodiment of the presentinvention, a light-emitting element including a novel compound can beprovided. According to one embodiment of the present invention, a novellight-emitting device with high emission efficiency and low powerconsumption can be provided. According to one embodiment of the presentinvention, a novel display device can be provided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are schematic cross-sectionals views of a light-emittingelement according to one embodiment of the present invention and FIG. 1Cis a schematic diagram illustrating the correlation of energy levels;

FIGS. 2A to 2C show calculation examples of compounds according to oneembodiment of the present invention;

FIG. 3 is a schematic cross-sectional view of a light-emitting elementaccording to one embodiment of the present invention;

FIG. 4 is a schematic cross-sectional view of a light-emitting elementaccording to one embodiment of the present invention;

FIGS. 5A and 5B are each a schematic cross-sectional view of alight-emitting element according to one embodiment of the presentinvention;

FIGS. 6A and 6B are a schematic cross-sectional view of a light-emittingelement according to one embodiment of the present invention and aschematic diagram illustrating the correlation of energy levels;

FIGS. 7A and 7B are a schematic cross-sectional view of a light-emittingelement according to one embodiment of the present invention and aschematic diagram illustrating the correlation of energy levels;

FIG. 8 is a schematic cross-sectional view of a semiconductor elementaccording to one embodiment of the present invention;

FIGS. 9A and 9B are a block diagram and a circuit diagram showing adisplay device according to one embodiment of the present invention;

FIGS. 10A and 10B are perspective views of an example of a touch panelaccording to one embodiment of the present invention;

FIGS. 11A to 11C are cross-sectional views of examples of the displaydevice and the touch sensor according to one embodiment of the presentinvention;

FIGS. 12A and 12B are cross-sectional views of examples of a touch panelaccording to one embodiment of the present invention;

FIGS. 13A and 13B are a block diagram and a timing chart of a touchsensor according to one embodiment of the present invention;

FIG. 14 is a circuit diagram of a touch sensor according to oneembodiment of the present invention;

FIG. 15 is a perspective view of a display module according to oneembodiment of the present invention;

FIGS. 16A to 16G illustrate electronic devices according to oneembodiment of the present invention; and

FIG. 17 illustrates lighting devices according to one embodiment of thepresent invention;

FIGS. 18A and 18B show NMR charts of compounds according to Example 1;

FIGS. 19A and 19B show absorption and emission spectra of the compoundaccording to Example 1;

FIGS. 20A and 20B show absorption and emission spectra of the compoundaccording to Example 1;

FIGS. 21A and 21B show fluorescence lifetime characteristics oflight-emitting elements according to Example 2;

FIG. 22 shows current efficiency-luminance characteristics of thelight-emitting elements according to Example 2;

FIG. 23 shows external quantum efficiency-luminance characteristics ofthe light-emitting elements according to Example 2; and

FIG. 24 shows luminance-voltage characteristics of the light-emittingelements according to Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be explained below withreference to the drawings. However, the present invention is not limitedto description to be given below, and it is to be easily understood thatmodes and details thereof can be variously modified without departingfrom the purpose and the scope of the present invention. Accordingly,the present invention should not be interpreted as being limited to thecontent of the embodiments below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different diagrams are commonly denoted by the samereference numeral in some cases.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other depending on the case or circumstances. Forexample, the term “conductive layer” can be changed into the term“conductive film” in some cases. Also, the term “insulating film” can bechanged into the term “insulating layer” in some cases.

In this specification and the like, a singlet excited state (S*) means asinglet state having excited energy. Among singlet excited states, anexcited state having the lowest energy is referred to as the lowestsinglet excited state. Furthermore, a singlet excited energy level meansan energy level in a singlet excited state. Among singlet excited energylevels, the lowest excited energy level is referred to as the lowestsinglet excited energy (S₁) level. Note that in this specification andthe like, simple expressions “singlet excited state” and “singletexcited energy level” mean the lowest singlet excited state and the S₁level, respectively, in some cases.

In this specification and the like, a triplet excited state (T*) means atriplet state having excited energy. Among triplet excited states, anexcited state having the lowest energy is referred to as the lowesttriplet excited state. Furthermore, a triplet excited energy level meansan energy level in a triplet excited state. Among triplet excited energylevels, the lowest excited energy level is referred to as the lowesttriplet excited energy (T₁) level. Note that in this specification andthe like, simple expressions “triplet excited state” and “tripletexcited energy level” mean the lowest triplet excited state and the T₁level, respectively, in some cases.

In this specification and the like, a fluorescent material refers to amaterial that emits light in the visible light region when the singletexcited state relaxes to the ground state. A phosphorescent materialrefers to a material that emits light in the visible light region atroom temperature when the triplet excited state relaxes to the groundstate. That is, a phosphorescent material refers to a material that canconvert triplet excited energy into visible light.

Note that in this specification and the like, room temperature refers toa temperature in the range from 0° C. to 40° C.

In this specification and the like, the blue wavelength range refers toa range in which the wavelength is greater than or equal to 400 nm andless than or equal to 550 nm, and the blue light emission refers tolight emission with at least one emission spectrum peak in the range.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described below with reference to FIGS. 1A and1B and FIGS. 2A to 2C.

Structure Example of Light-Emitting Element

First, a structure of a light-emitting element of one embodiment of thepresent invention is described below with reference to FIGS. 1A to 1C.

FIG. 1A is a schematic cross-sectional view of a light-emitting element150 of one embodiment of the present invention.

The light-emitting element 150 includes an EL layer 100 between a pairof electrodes (an electrode 101 and an electrode 102). The EL layer 100includes at least a light-emitting layer 130. Note that in thisembodiment, although description is given assuming that the electrode101 and the electrode 102 of the pair of electrodes serve as an anodeand a cathode, respectively, they can be interchanged for the structureof the light-emitting element 150.

The EL layer 100 illustrated in FIG. 1A includes a functional layer inaddition to the light-emitting layer 130. The functional layer includesa hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 118, and an electron-injection layer 119. Notethat the structure of the EL layer 100 is not limited to the structureillustrated in FIG. 1A, and a structure may be employed in which atleast one selected from the hole-injection layer 111, the hole-transportlayer 112, the electron-transport layer 118, and the electron-injectionlayer 119 is included. Alternatively, the EL layer 100 may include afunctional layer which is capable of lowering a hole injection barrieror an electron injection barrier, improving a hole-transport property oran electron-transport property, inhibiting a hole-transport property oran electron-transport property, or suppressing a quenching phenomenon byan electrode, for example.

FIG. 1B is a schematic cross-sectional view of an example of thelight-emitting layer 130 in FIG. 1A. The light-emitting layer 130 inFIG. 1B includes at least a host material 131 and a guest material 132.

The host material 131 preferably has a function of converting tripletexcited energy into singlet excited energy by causing TTA, so that thetriplet excited energy generated in the light-emitting layer 130 can bepartly converted into singlet excited energy by TTA in the host material131. The singlet excited energy generated by TTA can be transferred tothe guest material 132 and extracted as phosphorescence. In order toachieve this, the lowest singlet excited energy (S₁) level of the hostmaterial 131 is preferably higher than the S₁ level of the guestmaterial 132. In addition, the lowest triplet excited energy (T₁) levelof the host material 131 is preferably lower than the T₁ level of theguest material 132.

Note that the host material 131 may be composed of a single compound ora plurality of compounds. The guest material 132 may be a light-emittingorganic material, and the light-emitting organic material is preferablya material capable of emitting fluorescence (hereinafter also referredto as a fluorescent material). A fluorescent material is used as theguest material 132. Note that the guest material 132 may be read as thefluorescent material.

<Emission Mechanism of Light-Emitting Element>

First, an emission mechanism of the light-emitting element 150 isdescribed below.

In the light-emitting element 150 of one embodiment of the presentinvention, voltage application between a pair of electrodes (electrodes101 and 102) causes electrons and holes to be injected from the cathodeand the anode, respectively, into the EL layer 100 and current flows. Byrecombination of the injected electrons and holes, excitons are formed.The ratio of singlet excitons to triplet excitons in the excitons formedby the carrier recombination is 1:3 according to the statisticallyobtained probability. Therefore, the probability of formation of singletexcitons is 25%.

Note that the term “exciton” refers to a pair of carriers (an electronand a hole). Since an exciton has excited energy, a material where anexciton is formed is brought into an excited state.

Through two processes described below, singlet excitons are formed inthe EL layer 100 and light emission from the guest material 132 can beobtained. The processes are (α) direct formation process and (β) TTAprocess.

<<(α) Direct Formation Process>>

The case where carriers (electrons and holes) recombine in thelight-emitting layer 130 included in the EL layer 100 to form a singletexciton is first described.

When the carriers recombine in the host material 131, excitons areformed to bring the host material 131 into an excited state (a singletexcited state or a triplet excited state). At this time, in the casewhere the excited state of the host material 131 is a singlet excitedstate, singlet excited energy transfers from the S₁ level of the hostmaterial 131 to the S₁ level of the guest material 132, thereby formingthe singlet excited state of the guest material 132. Note that the casewhere the excited state of the host material 131 is a triplet excitedstate is described in later in (β) TTA process.

When the carriers recombine in the guest material 132, excitons areformed to bring the guest material 132 into an excited state (a singletexcited state or a triplet excited state).

In the case where the formed excited state of the guest material 132 isa singlet excited state, light emission is obtained from the singletexcited state of the guest material 132. To obtain high emissionefficiency in this case, the fluorescence quantum yield of the guestmaterial 132 is preferably high.

In the case where the formed excited state of the guest material 132 isa triplet excited state, the triplet excited state of the guest material132 is thermally deactivated and does not contribute to light emissionbecause the guest material 132 is a fluorescent material. However, ifthe T₁ level of the host material 131 is lower than the T₁ level of theguest material 132, the triplet excited energy of the guest material 132can be transferred from the T₁ level of the guest material 132 to the T₁level of the host material 131. In this case, the triplet excited energycan be converted into singlet excited energy by (β) TTA processdescribed later.

In the case where the T₁ level of the host material 131 is higher thanthe T₁ level of the guest material 132, if the weight ratio of the guestmaterial 132 to the host material 131 is low, the probability of carrierrecombination in the guest material 132 can be reduced. In addition, theprobability of energy transfer from the T₁ level of the host material131 to the T₁ level of the guest material 132 can be reduced.Specifically, the weight ratio of the guest material 132 to the hostmaterial 131 is preferably greater than 0 and less than or equal to0.05.

<<(β) TTA Process>>

The case where a singlet exciton is formed from triplet excitons formedin the carrier recombination process in the light-emitting layer 130 isdescribed.

Here, the case where the T₁ level of the host material 131 is lower thanthe T₁ level of the guest material 132 is described. The correlation ofenergy levels in this case is schematically shown in FIG. 1C. What termsand signs in FIG. 1C represent are listed below. Note that the T₁ levelof the host material 131 may be higher than the T₁ level of the guestmaterial 132.

Host (131): the host material 131

Guest (132): the guest material 132 (fluorescent material)

S_(FH): the S₁ level of the host material 131

T_(FH): the T₁ level of the host material 131

S_(FG): the S₁ level of the guest material 132 (fluorescent material)

T_(FG): the T₁ level of the guest material 132 (fluorescent material)

Carriers recombine in the host material 131 and excitons are formed tobring the host material 131 into an excited state. At this time, in thecase where the formed excitons are triplet excitons and two of theformed triplet excitons approach each other, a reaction in which one ofthem is converted into a singlet exciton having energy of the S₁ level(S_(FH)) of the host material 131 might be caused (see TTA in FIG. 1C).This is represented by the following general formula (G11) or generalformula (G12).

³H+³H→¹H*+¹H  (G11)

³H+³H→³H*+¹H  (G12)

The general formula (G11) represents a reaction in the host material 131in which a singlet exciton (¹H*) is formed from two triplet excitons(³H) with a total spin quantum number of 0. The general formula (G12)represents a reaction in the host material 131 in which anelectronically or oscillatorily excited triplet exciton (³H*) is formedfrom two triplet excitons (³H) with a total spin quantum number of 1(atomic unit). In the general formulae (G11) and (G12), ¹H representsthe singlet ground state of the host material 131.

Although the reactions in the general formulae (G11) and (G12) occur atthe same probability, there are three times as many pairs of tripletexcitons with a total spin quantum number of 1 (atomic unit) as pairs oftriplet excitons with a total spin quantum number of 0. In other words,when an exciton is formed from two triplet excitons, the singlet-tripletexciton formation ratio is 1:3 according to the statistically obtainedprobability. In the case where the density of the triplet excitons inthe light-emitting layer 130 is sufficiently high (e.g., 1×10⁻¹² cm⁻³ ormore), only the reaction of two triplet excitons approaching each othercan be considered whereas deactivation of a single triplet exciton isignored.

Thus, by one reaction in the general formula (G11) and three reactionsin the general formula (G12), one singlet exciton (¹H*) and threetriplet excitons (³H*) which are electronically or oscillatorily excitedare formed from eight triplet excitons (³H). This is represented by ageneral formula (G13).

8³H→¹H*+3³H*+4¹H  (G13)

The electronically or oscillatorily excited triplet excitons (³H*),which are formed as in the general formula (G13), become tripletexcitons (³H) by relaxation and then repeat the reaction in the generalformula (G13) again with other triplet excitons. Hence, in the generalformula (G13), if all the triplet excitons (³H) are converted intosinglet excitons (¹H*), five triplet excitons (³H) form one singletexciton (¹H*) (a general formula (G14)).

5³H→¹H*+4¹H  (G14)

The ratio of singlet excitons (¹H*) to triplet excitons (³H) which aredirectly formed by recombination of carriers injected from a pair ofelectrodes is statistically as follows: ¹H*:³H=1:3. That is, theprobability of singlet excitons being directly formed by recombinationof carriers injected from a pair of electrodes is 25%.

When the singlet excitons directly formed by recombination of carriersinjected from a pair of electrodes and the singlet excitons formed byTTA are put together, eight singlet excitons can be formed from twentyexcitons (the sum of singlet excitons and triplet excitons) directlyformed by recombination of carriers injected from a pair of electrodes(a general formula (G15)). That is, TTA can increase the probability ofsinglet exciton formation from 25%, which is the conventional value, toat most 40% (=8/20).

5¹H*+15³H→5¹H*+(3¹H*+12¹H)  (G15)

In the singlet excited state of the host material 131 which is formed bythe singlet excitons formed by the above process, energy is transferredfrom the S₁ level (S_(FH)) of the host material 131 to the S₁ level(S_(FG)) of the guest material 132, which is lower than S_(FH) (seeRoute A in FIG. 1C). Then, the guest material 132 brought into a singletexcited state emits fluorescence.

In the case where carriers recombine in the guest material 132 and anexcited state formed by the formed excitons is a triplet excited state,if the T₁ level (T_(FH)) of the host material 131 is lower than the T₁level (T_(FG)) of the guest material, triplet excited energy of T_(FG)is not deactivated and transferred to T_(FH) (see Route B in FIG. 1C) tocontribute to TTA.

In the case where the T₁ level (T_(FG)) of the guest material 132 islower than the T₁ level (T_(FH)) of the host material 131, the weightratio of the guest material 132 to the host material 131 is preferablylow. Specifically, the weight ratio of the guest material 132 to thehost material 131 is preferably greater than 0 and less than or equal to0.05, in which case, the probability of carrier recombination in theguest material 132 can be reduced. In addition, the probability ofenergy transfer from the T₁ level (T_(FH)) of the host material 131 tothe T₁ level (T_(FG)) of the guest material 132 can be reduced.

As described above, triplet excitons formed in the light-emitting layer130 can be converted into singlet excitons by TTA, so that light emittedfrom the guest material 132 can be efficiently obtained.

<TTA Efficiency>

Since TTA can increase the probability of formation of singlet excitonsand the emission efficiency of a light-emitting element as describedabove, increasing the probability of occurrence of TTA (also referred toas TTA efficiency) is important for high emission efficiency. That is,it is important that a delayed fluorescence component due to TTA accountfor a large proportion of light emitted from the light-emitting element.

As described above, owing to the TTA process, the probability offormation of singlet excitons can be increased to at most 40% including25% occupied by the singlet excitons that are directly formed byrecombination of carriers injected from a pair of electrodes. Thus, adelayed fluorescence component due to TTA can account for at most 37.5%((40%−25%)/40%) of light emitted from the light-emitting element.

Anthracene compounds are generally used as a host material in alight-emitting element that emits blue light, and a delayed fluorescencecomponent due to TTA accounts for approximately 10% of light emission.In contrast, tetracene compounds known as a compound with high TTAefficiency are difficult to use as a host material in a light-emittingelement that emits blue light because they are compounds that emityellow light or light with a longer wavelength than yellow light.

Thus, to achieve a light-emitting element that emits blue light and hashigh emission efficiency in which a delayed fluorescence component dueto TTA accounts for a large proportion, the use of a compound in which adelayed fluorescence component due to TTA accounts for a largeproportion as in tetracene and which has high excited energy likeanthracene, as a host material, is required.

<Quantum Chemical Calculations>

Quantum chemical calculations were performed to find the excited energylevels (S₁ level and T₁ level) and the transition dipole momentum andoscillator strengths from the singlet ground state to the lowest singletexcited state of anthracene, which is a tricyclic aromatic hydrocarbon,and tetracene and benzo[a]anthracene, which are tetracyclic aromatichydrocarbons. The compounds used for the calculations are shown in FIGS.2A to 2C and the calculation results are listed in Table 1. Note thatFIG. 2A shows anthracene, FIG. 2B tetracene, and FIG. 2Cbenzo[a]anthracene. The calculation method is described below.

TABLE 1 S₁ level T₁ level Transition dipole moment OscillatorAbbreviation (eV) (eV) x y z strength Anthracene 3.24 1.84 0 0.846 00.0568 Tetracene 2.46 1.16 0 0.893 0 0.0481 Benzo[a]anthracene 3.36 2.08−0.436 −0.651 0 0.0506

To find the S₁ levels, T₁ levels, transition dipole momentum, andoscillator strengths of the above compounds, the most stable structurein the singlet ground state of each compound was calculated using thedensity functional theory (DFT). As the quantum chemistry computationalprogram, Gaussian 09 was used. A high performance computer (manufacturedby SGI Japan, Ltd.) was used for the calculation. As a basis function,6-311G (d,p) was used, and as a functional, B3LYP was used. Thetime-dependent density functional theory (TD-DFT) was further used tofind the S₁ level, the T₁ level, and the transition dipole moment fromthe singlet ground state to the lowest singlet excited state. In theDFT, the total energy is represented as the sum of potential energy,electrostatic energy between electrons, electronic kinetic energy, andexchange-correlation energy including all the complicated interactionsbetween electrons. Also in the DFT, an exchange-correlation interactionis approximated by a functional (a function of another function) of oneelectron potential represented in terms of electron density to enablehigh-accuracy calculations.

As listed in Table 1, benzo[a]anthracene is a tetracyclic aromatichydrocarbon but has a S₁ level and a T₁ level that are approximately ashigh as those of anthracene, which is a tricyclic aromatic hydrocarbon.

The transition dipole moment of benzo[a]anthracene is approximately ashigh as those of anthracene and tetracene, and the oscillator strengthof benzo[a]anthracene is also sufficiently high. The transition dipolemomentum and oscillator strengths shown in Table 1 relate to thetransition from the singlet ground state to the lowest singlet excitedstate in the compounds. The arrows in FIGS. 2A to 2C indicate thedirections (x, y, and z) of the transition dipole momentum. Thez-direction is perpendicular to the paper. The reason for the negativevalue of the transition dipole moment of benzo[a]anthracene is becausethe directions of the transition dipole moment are opposite to the x-and y-directions in FIGS. 2A to 2C. Since the magnitude of thetransition dipole moment can be evaluated from its absolute value, thetransition dipole moment of benzo[a]anthracene can be found sufficientlylarge.

A large transition dipole moment and high oscillator strength from thesinglet ground state to the lowest singlet excited state mean that thelowest singlet excited state can be easily generated. Thus, the lowestsinglet excited states of anthracene, tetracene, and benzo[a]anthracenecan be easily generated, and therefore these compounds are likely toexhibit high TTA efficiency in a TTA process.

In particular, a tetracyclic aromatic hydrocarbon compound (e.g.,benzo[a]anthracene), which is a tetracyclic aromatic hydrocarboncompound like tetracene and has a nonlinear structure with approximatelyas high singlet excited energy as that of anthracene, has high excitedenergy and is likely to exhibit high TTA efficiency. Thus, in thecompound, the proportion of a delayed fluorescence component due to TTAcan be increased.

When higher emission efficiency than that of a light-emitting elementincluding an anthracene compound as a host material is desired, theproportion of a delayed fluorescence component due to TTA is preferablylarger than that in the light-emitting element including an anthracenecompound as a host material. Specifically, a delayed fluorescencecomponent due to TTA preferably accounts for 20% or more of emissivecomponents of the light-emitting element.

An emission spectrum of a light-emitting element preferably has a peakin the blue wavelength range, or more specifically, at least one peak ata wavelength greater than or equal to 400 nm and less than or equal to550 nm.

Note that a factor of delayed fluorescence in a light-emitting element,which is other than TTA, may be thermally activated delayed fluorescencedue to reverse intersystem crossing from the triplet excited state tothe singlet excited state. To efficiently cause reverse intersystemcrossing, an energy difference between the S₁ level and the T₁ level ispreferably 0.2 eV or less. In other words, an energy difference greaterthan 0.2 eV between the S₁ level and the T₁ level hardly causes reverseintersystem crossing. Therefore, to efficiently cause TTA, an energydifference between the lowest singlet excited energy level and lowesttriplet excited energy level of a compound in which TTA occurs ispreferably greater than 0.2 eV, further preferably greater than or equalto 0.5 eV

The lowest singlet excited energy level of an organic compound can beobserved from an absorption spectrum at a transition from the singletground state to the lowest singlet excited state in the organiccompound. Alternatively, the lowest singlet excited energy level may beestimated from a peak wavelength of a fluorescence spectrum of theorganic compound. Furthermore, the lowest triplet excited energy levelcan be observed from an absorption spectrum at a transition from thesinglet ground state to the lowest triplet excited state in the organiccompound, but is difficult to observe in some cases because thistransition is a forbidden transition. In such cases, the lowest tripletexcited energy level may be estimated from a peak wavelength of aphosphorescence spectrum of the organic compound. Thus, a difference inequivalent energy value between the peak wavelengths of the fluorescenceand phosphorescence spectra of the organic compound is preferablygreater than 0.2 eV, further preferably greater than or equal to 0.5 eV

<Materials>

Next, components of a light-emitting element of one embodiment of thepresent invention are described in detail.

<<Light-Emitting Layer>>

In the light-emitting layer 130, the weight percentage of the hostmaterial 131 is higher than that of at least the guest material 132, andthe guest material 132 (fluorescent material) is dispersed in the hostmaterial 131. A material that can be used as the host material 131 inthe light-emitting layer 130 is preferably an organic compound in whicha delayed fluorescence component due to triplet-triplet annihilation(TTA) accounts for a large proportion of emitted light, or morespecifically, an organic compound in which a delayed fluorescencecomponent due to TTA accounts for 20% or more. In particular, a compoundhaving a benzo[a]anthracene skeleton is preferred. In the light-emittinglayer 130, the host material 131 may be composed of one kind of compoundor a plurality of compounds.

In the light-emitting layer 130, the guest material 132 is preferably,but not particularly limited to, an anthracene derivative, a tetracenederivative, a chrysene derivative, a phenanthrene derivative, a pyrenederivative, a perylene derivative, a stilbene derivative, an acridonederivative, a coumarin derivative, a phenoxazine derivative, aphenothiazine derivative, or the like, and for example, any of thefollowing materials can be used.

The examples include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N′-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 6, coumarin 545T,N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-m]perylene.

The light-emitting layer 130 may include a material other than the hostmaterial 131 and the guest material 132.

Although there is no particular limitation on a material that can beused in the light-emitting layer 130, any of the following materials canbe used, for example: metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11); and aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB ora-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). In addition, condensed polycyclic aromaticcompounds such as anthracene derivatives, phenanthrene derivatives,pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysenederivatives can be used. Specific examples thereof include9,10-diphenylanthracene (abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), and1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3). A plurality ofcompounds each having a benzo[a]anthracene skeleton may be included. Oneor more substances having a wider energy gap than the guest material 132is preferably selected from these substances and known substances.

The light-emitting layer 130 can have a structure in which two or morelayers are stacked. For example, in the case where the light-emittinglayer 130 is formed by stacking a first light-emitting layer and asecond light-emitting layer in this order from the hole-transport layerside, the first light-emitting layer is formed using a substance havinga hole-transport property as the host material and the secondlight-emitting layer is formed using a substance having anelectron-transport property as the host material. Also in such a case,at least one light-emitting layer preferably includes a compound havinga benzo[a]anthracene skeleton.

Next, details of other components of the light-emitting element 150 inFIG. 1A are described.

<<Pair of Electrodes>>

The electrode 101 and the electrode 102 have functions of injectingholes and electrons into the light-emitting layer 130. The electrodes101 and 102 can be formed using a metal, an alloy, or a conductivecompound, or a mixture or a stack thereof, for example. A typicalexample of the metal is aluminum, besides, a transition metal such assilver, tungsten, chromium, molybdenum, copper, or titanium, an alkalimetal such as lithium or cesium, or a Group 2 metal such as calcium ormagnesium can be used. As the transition metal, a rare earth metal suchas ytterbium (Yb) may be used. An alloy containing any of the abovemetals can be used as the alloy, and MgAg and AlLi can be given asexamples. As the conductive compound, a metal oxide such as indiumoxide-tin oxide (indium tin oxide) can be given. It is also possible touse an inorganic carbon-based material such as graphene as theconductive compound. As described above, the electrode 101 and/or theelectrode 102 may be formed by stacking two or more of these materials.

Light emitted from the light-emitting layer 130 is extracted through theelectrode 101 and/or the electrode 102. Therefore, at least one of theelectrodes 101 and 102 transmits visible light. In the case where theelectrode through which light is extracted is formed using a materialwith low light transmittance, such as metal or alloy, the electrode 101and/or the electrode 102 is formed to a thickness that is thin enough totransmit visible light (e.g., a thickness of 1 nm to 10 nm).

<<Hole-Injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier forhole injection from one of the pair of electrodes (the electrode 101 orthe electrode 102) to promote hole injection and is formed using atransition metal oxide, a phthalocyanine derivative, or an aromaticamine, for example. As the transition metal oxide, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be given. As the phthalocyanine derivative, phthalocyanine,metal phthalocyanine, or the like can be given. As the aromatic amine, abenzidine derivative, a phenylenediamine derivative, or the like can begiven. It is also possible to use a high molecular compound such aspolythiophene or polyaniline; a typical example thereof ispoly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which isself-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite materialof a hole-transport material and a material having a property ofaccepting electrons from the hole-transport material can also be used.Alternatively, a stack of a layer containing a material having anelectron accepting property and a layer containing a hole-transportmaterial may also be used. In a steady state or in the presence of anelectric field, electric charge can be transferred between thesematerials. As examples of the material having an electron-acceptingproperty, organic acceptors such as a quinodimethane derivative, achloranil derivative, and a hexaazatriphenylene derivative can be given.A specific example is a compound having an electron-withdrawing group (ahalogen group or a cyano group), such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, or2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN). Alternatively, a transition metal oxide such as an oxide of ametal from Group 4 to Group 8 can also be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.In particular, molybdenum oxide is preferable because it is stable inthe air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, anaromatic amine, a carbazole derivative, an aromatic hydrocarbon, astilbene derivative, or the like can be used. Furthermore, thehole-transport material may be a high molecular compound.

Examples of the material having a high hole-transport property areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivative are4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon are2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Other examples are pentacene, coronene, and the like. Thearomatic hydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or more andhaving 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon that can be used for the composite material mayhave a vinyl skeleton. Examples of the aromatic hydrocarbon having avinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

<<Hole-Transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transportmaterial and can be formed using any of the materials given as examplesof the material of the hole-injection layer 111. In order that thehole-transport layer 112 has a function of transporting holes injectedinto the hole-injection layer 111 to the light-emitting layer 130, thehighest occupied molecular orbital (HOMO) level of the hole-transportlayer 112 is preferably equal or close to the HOMO level of thehole-injection layer 111.

In addition to the hole-transport materials given as the material of thehole-injection layer 111, examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances described here aremainly substances having a hole mobility of 1×10⁻⁶ cm²/Vs or higher.Note that any substance other than the above substances may be used aslong as the hole-transport property is higher than theelectron-transport property. The layer including a substance having ahigh hole-transport property is not limited to a single layer, and twoor more layers containing the aforementioned substances may be stacked.

The compound having a benzo[a]anthracene skeleton may also be used as amaterial contained in the hole-transport layer 112.

<<Electron-Transport Layer>>

The electron-transport layer 118 has a function of transporting, to thelight-emitting layer 130, electrons injected from the other of the pairof electrodes (the electrode 101 or the electrode 102) through theelectron-injection layer 119. A material having a property oftransporting more electrons than holes can be used as anelectron-transport material, and a material having an electron mobilityof 1×10⁻⁶ cm²/Vs or higher is preferable. Specific examples include ametal complex having a quinoline ligand, a benzoquinoline ligand, anoxazole ligand, or a thiazole ligand; an oxadiazole derivative; atriazole derivative; a phenanthroline derivative; a pyridine derivative;a bipyridine derivative; a pyrimidine derivative.

For example, the electron-transport layer is formed using a metalcomplex having a quinoline skeleton or a benzoquinoline skeleton, suchas tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(III) (abbreviation: BeBq₂),or bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), or the like. A metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc(II) (abbreviation: Zn(BOX)₂)or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation:Zn(BTZ)₂), or the like can also be used. Other than the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances given here are mainly ones having an electron mobility of10⁻⁶ cm²/Vs or higher. A compound having a benzo[a]anthracene skeletoncan also be suitably used. Note that any substance other than the abovesubstances may be used for the electron-transport layer as long as thesubstance has an electron-transport property higher than ahole-transport property. Furthermore, the electron-transport layer 118is not limited to a single layer and may be a stack of two or morelayers including any of the above substances.

Between the electron-transport layer 118 and the light-emitting layer130, a layer that controls transport of electron carriers may beprovided. This is a layer formed by addition of a small amount of asubstance having a high electron-trapping property to the aforementionedmaterial having a high electron-transport property, and the layer iscapable of adjusting carrier balance by retarding transport of electroncarriers. Such a structure is very effective in preventing a problem(such as a reduction in element lifetime) caused when electrons passthrough the light-emitting layer.

<<Electron-Injection Layer>>

The electron-injection layer 119 has a function of reducing a barrierfor electron injection from the electrode 102 to promote electroninjection and can be formed using a Group 1 metal or a Group 2 metal, oran oxide, a halide, or a carbonate of any of the metals, for example.Alternatively, a composite material containing an electron-transportmaterial (described above) and a material having a property of donatingelectrons to the electron-transport material can also be used. As thematerial having an electron-donating property, a Group 1 metal, a Group2 metal, an oxide of any of the metals, or the like can be given.

Note that the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer described above can each be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, a gravure printing method, or the like.Besides the above-mentioned materials, an inorganic compound or a highmolecular compound (e.g., an oligomer, a dendrimer, or a polymer) may beused in the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer.

<<Substrate>>

The light-emitting element 150 is fabricated over a substrate of glass,plastic, or the like. As the way of stacking layers over the substrate,layers may be sequentially stacked from the electrode 101 side orsequentially stacked from the electrode 102 side.

Note that, for example, glass, quartz, plastic, or the like can be usedfor the substrate over which the light-emitting element 150 can beformed. Alternatively, a flexible substrate can be used. The flexiblesubstrate is a substrate that can be bent, such as a plastic substratemade of polycarbonate or polyarylate, for example. A film, an inorganicfilm formed by evaporation, or the like can also be used. Note thatmaterials other than these can be used as long as they can function as asupport in a manufacturing process of the light-emitting element and anoptical element or as long as they have a function of protecting thelight-emitting element and the optical element.

The light-emitting element 150 can be formed using a variety ofsubstrates, for example. The type of substrate is not limited to acertain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used, for example.Examples of the glass substrate include a barium borosilicate glasssubstrate, an aluminoborosilicate glass substrate, and a soda lime glasssubstrate. Examples of the flexible substrate, the attachment film, thebase material film, and the like are substrates of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Anotherexample is a resin such as acrylic. Other examples are polypropylene,polyester, polyvinyl fluoride, polyvinyl chloride, and the like. Otherexamples are polyamide, polyimide, aramid, epoxy, an inorganic filmformed by evaporation, paper, and the like.

Alternatively, a flexible substrate may be used as the substrate, andthe light-emitting element may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate and the light-emitting element. The separation layer can beused when part or the whole of the light-emitting element formed overthe separation layer is completed, separated from the substrate, andtransferred to another substrate. In such a case, the light-emittingelement can be transferred to a substrate having low heat resistance ora flexible substrate as well. For the above separation layer, a stackincluding inorganic films, which are a tungsten film and a silicon oxidefilm, or a resin film of polyimide or the like formed over a substratecan be used, for example.

In other words, after the light-emitting element is formed using asubstrate, the light-emitting element may be transferred to anothersubstrate. Examples of a substrate to which the light-emitting elementis transferred include, in addition to the above-described substrates, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester, or thelike), a leather substrate, and a rubber substrate. By using such asubstrate, a light-emitting element with high durability, alight-emitting element with high heat resistance, a lightweightlight-emitting element, or a thin light-emitting element can beobtained.

The light-emitting element 150 may be formed over an electrodeelectrically connected to a field-effect transistor (FET), for example,which is formed over the above-mentioned substrate, so that an activematrix display device in which the FET controls the drive of thelight-emitting element 150 can be manufactured.

In Embodiment 1, one embodiment of the present invention is described.Other embodiments of the present invention are described in the otherembodiments. Note that one embodiment of the present invention is notlimited thereto. Although the case where a delayed fluorescencecomponent due to triplet-triplet annihilation accounts for 20% or moreof light emitted from the EL layer and the emitted light has an emissionspectrum peak in the blue wavelength range is exemplified in oneembodiment of the present invention, one embodiment of the presentinvention is not limited thereto. Depending on circumstances orconditions, in one embodiment of the present invention, a delayedfluorescence component need not account for 20% or more of light emittedfrom the EL layer. Alternatively, the emitted light need not have anemission spectrum peak in the blue wavelength range. Alternatively, theemitted light need not have any emission spectrum peak at a wavelengthgreater than or equal to 400 nm and less than or equal to 550 nm.Although the case where the EL layer includes an organic compound withan energy difference of 0.5 eV or more between the lowest singletexcited energy level and the lowest triplet excited energy level, adelayed fluorescence component accounts for 20% or more of light emittedfrom the EL layer, and the emitted light has an emission spectrum peakin the blue wavelength range is exemplified in one embodiment of thepresent invention, one embodiment of the present invention is notlimited thereto. Depending on circumstances or conditions, in oneembodiment of the present invention, the EL layer need not include anorganic compound with an energy difference of 0.5 eV or more between thelowest singlet excited energy level and the lowest triplet excitedenergy level. Although the case where the EL layer includes a compoundhaving a benzo[a]anthracene skeleton and a delayed fluorescencecomponent accounts for 20% or more of light emitted from the EL layer isexemplified in one embodiment of the present invention, one embodimentof the present invention is not limited thereto. Depending oncircumstances, the EL layer need not include a compound having abenzo[a]anthracene skeleton.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, a compound that can suitably be used in alight-emitting element of one embodiment of the present invention isdescribed below.

A compound of this embodiment includes a carbazole derivative in which acarbazole skeleton is bonded to the 7-position of a benzo[a]anthraceneskeleton via at least an arylene group. Since a delayed fluorescencecomponent due to TTA accounts for a large proportion of emitted light inthe benzo[a]anthracene compound, use of the benzo[a]anthracene compoundin a light-emitting element achieves high emission efficiency. The wideband gap of the benzo[a]anthracene compound also leads to high emissionefficiency of the light-emitting element using the benzo[a]anthracenecompound. In particular, a light-emitting element with high emissionefficiency which emits blue light can be fabricated. Furthermore,because of the excellent carrier-transport property of thebenzo[a]anthracene compound, use of the benzo[a]anthracene compound in alight-emitting element enables the light-emitting element to be drivenat a low voltage. In addition, since the benzo[a]anthracene compound ishighly resistant to repetition of oxidation and reduction, use of thebenzo[a]anthracene compound in a light-emitting element enables the longoperating life of the light-emitting element. As described above, theuse of the compound of this embodiment in a light-emitting elementachieves a high-performance light-emitting element having excellentemission characteristics.

When the number of carbon atoms of the arylene group is 6 to 13, thebenzo[a]anthracene compound is a low molecular compound with arelatively low molecular weight and accordingly has a structure suitablefor vacuum evaporation (capable of being vacuum-evaporated at arelatively low temperature). In general, a lower molecular weight tendsto diminish heat resistance after film formation. However, even with alow molecular weight, the benzo[a]anthracene compound has an advantagein that sufficient heat resistance can be ensured because of the effectof the rigid benzo[a]anthracene skeleton.

Since the carbazole skeleton is bonded to the 7-position of abenzo[a]anthracene skeleton via an arylene group, the carrier-transportproperty of the benzo[a]anthracene compound is improved. Accordingly, alight-emitting element using the benzo[a]anthracene compound can bedriven at a low voltage.

The above benzo[a]anthracene compound can also be referred to as abenzo[a]anthracene compound in which an arylcarbazole derivative isbonded to a benzo[a]anthracene skeleton. The benzo[a]anthracene compoundcan be easily synthesized with high purity, so that deterioration due toimpurities can be suppressed. The number of carbon atoms of the arylgroup of the arylcarbazole derivative which is bonded to thebenzo[a]anthracene skeleton is preferably 6 to 13 in terms of thestability and reliability of element characteristics. In this case, thebenzo[a]anthracene compound can be vacuum-evaporated at a relatively lowtemperature as described above and accordingly is unlikely todeteriorate due to pyrolysis or the like at evaporation. In addition,the compound is excellent in not only reliability but also drivevoltage. This is also because the benzo[a]anthracene compound has highelectrochemical stability and a high carrier-transport property owing tothe molecular structure in which a carbazole skeleton is bonded to the7-position of a benzo[a]anthracene skeleton via an arylene group.

A benzo[a]anthracene compound in which a benzo[a]anthracene skeleton isbonded to the 9-position of a carbazole skeleton via an arylene grouphas a wide band gap, and therefore can suitably be used particularly ina light-emitting element that emits light with high energy such as bluelight. Note that the carbazole skeleton and the benzo[a]anthraceneskeleton are preferably bonded via an arylene group such as a phenylenegroup or a naphthylene group.

For the above reason, the benzo[a]anthracene compound in which the9-position of a carbazole skeleton and the 7-position of abenzo[a]anthracene skeleton are bonded via an arylene group ispreferred. In other words, a benzo[a]anthracene compound in which a(9-carbazolyl)aryl group is bonded to the 7-position of abenzo[a]anthracene skeleton is preferred. The number of carbon atoms ofan aryl group of the (9-carbazolyl)aryl group which is bonded to thebenzo[a]anthracene skeleton is preferably 6 to 13 in terms of thestability of the compound and the light-emitting element. Thus, thebenzo[a]anthracene compound has a wide band gap which is a feature dueto the effect of the skeleton of a 9-carbazolyl group, in addition tothe high suitability for evaporation, electrochemical stability, andcarrier-transport property described above. Hence, thebenzo[a]anthracene compound is effective in a structure of alight-emitting element in which it is used as a host material of alight-emitting layer and a light-emitting material is added as a guestmaterial to the light-emitting layer. This compound is suitably used asa host material particularly in a blue light-emitting element.

Example 1 of Compound

The above-described benzo[a]anthracene compound is thebenzo[a]anthracene compound represented by the following general formula(G1).

In the above general formula (G1), A represents a substituted orunsubstituted carbazolyl group. In the case where the carbazolyl grouphas a substituent, as the substituent, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or asubstituted or unsubstituted aryl group having 6 to 12 carbon atoms canalso be selected. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 12 carbon atoms are a phenylgroup, a naphthyl group, a biphenyl group, and the like.

Furthermore, R¹ to R¹⁰ each independently represent any of hydrogen, analkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Furthermore, R¹¹ represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted phenyl group. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms are a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like. The above aryl group orphenyl group may include one or more substituents, and the substituentsmay be bonded to each other to form a ring. As the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, or an aryl group having 6 to 12 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 12 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, and the like.

Furthermore, Ar represents an arylene group having 6 to 13 carbon atoms.The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring. For example, acarbon atom at the 9-position in a fluorenyl group has two phenyl groupsas substituents and the phenyl groups are bonded to form a spirofluoreneskeleton. Specific examples of the arylene group having 6 to 13 carbonatoms are a phenylene group, a naphthylene group, a biphenylene group, afluorenediyl group, and the like. In the case where the arylene grouphas a substituent, as the substituent, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or an arylgroup having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms are a phenyl group, a naphthyl group, abiphenyl group, and the like.

Example 2 of Compound

As a benzo[a]anthracene compound of this embodiment, abenzo[a]anthracene compound having a structure in which abenzo[a]anthracene skeleton is bonded to the 9-position of a carbazolylgroup via an arylene group has a wide band gap, and therefore cansuitably be used especially in a light-emitting element that emits lightwith high energy such as blue light, which is preferable. Because thebenzo[a]anthracene compound has an excellent carrier-transport property,a light-emitting element including the compound can be driven at a lowvoltage, which is preferable. The above-described benzo[a]anthracenecompound is the benzo[a]anthracene compound represented by the followinggeneral formula (G2).

In the above general formula (G2), R¹ to R¹⁰ and R²¹ to R²⁸ eachindependently represent any of hydrogen, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.Furthermore, R¹¹ represents any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted phenyl group. Specific examples of thealkyl group having 1 to 6 carbon atoms include a methyl group, an ethylgroup, a propyl group, an isopropyl group, a butyl group, an isobutylgroup, a tert-butyl group, an n-hexyl group, and the like. Specificexamples of a cycloalkyl group having 3 to 6 carbon atoms include acyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexylgroup, and the like. Specific examples of the aryl group having 6 to 13carbon atoms are a phenyl group, a naphthyl group, a biphenyl group, afluorenyl group, and the like. The above aryl group or phenyl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or anaryl group having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, and the like.

Furthermore, Ar represents an arylene group having 6 to 13 carbon atoms.The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring. For example, acarbon atom at the 9-position in a fluorenyl group has two phenyl groupsas substituents and the phenyl groups are bonded to form a spirofluoreneskeleton. Specific examples of the arylene group having 6 to 13 carbonatoms are a phenylene group, a naphthylene group, a biphenylene group, afluorenediyl group, and the like. In the case where the arylene grouphas a substituent, as the substituent, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or an arylgroup having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms are a phenyl group, a naphthyl group, abiphenyl group, and the like.

Example 3 of Compound

As a benzo[a]anthracene compound of this embodiment, abenzo[a]anthracene compound having a structure in which abenzo[a]anthracene skeleton is bonded to any of the 1- to 4-positions ofa carbazolyl group via an arylene group has an excellentcarrier-transport property, and therefore a light-emitting elementincluding the compound can be driven at a low voltage, which ispreferable. The above-described benzo[a]anthracene compound is thebenzo[a]anthracene compound represented by the following general formula(G3).

In the above general formula (G3), R¹ to R¹⁰ and R³¹ to R³⁵ eachindependently represent any of hydrogen, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.Furthermore, R¹¹ represents any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted phenyl group. Specific examples of thealkyl group having 1 to 6 carbon atoms include a methyl group, an ethylgroup, a propyl group, an isopropyl group, a butyl group, an isobutylgroup, a tert-butyl group, an n-hexyl group, and the like. Specificexamples of a cycloalkyl group having 3 to 6 carbon atoms include acyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexylgroup, and the like. Specific examples of the aryl group having 6 to 13carbon atoms are a phenyl group, a naphthyl group, a biphenyl group, afluorenyl group, and the like. The above aryl group or phenyl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or anaryl group having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, and the like.

Furthermore, Ar represents an arylene group having 6 to 13 carbon atoms.The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring. For example, acarbon atom at the 9-position in a fluorenyl group has two phenyl groupsas substituents and the phenyl groups are bonded to form a spirofluoreneskeleton. Specific examples of the arylene group having 6 to 13 carbonatoms are a phenylene group, a naphthylene group, a biphenylene group, afluorenediyl group, and the like. In the case where the arylene grouphas a substituent, as the substituent, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or an arylgroup having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms are a phenyl group, a naphthyl group, abiphenyl group, and the like.

As a benzo[a]anthracene compound of this embodiment, abenzo[a]anthracene compound having a structure in which a carbazolylgroup is bonded to a benzo[a]anthracene skeleton via a substituted orunsubstituted phenylene group or a substituted or unsubstitutedbiphenyldiyl group can have improved stability and be synthesized withhigher purity, which is preferable. Because the benzo[a]anthracenecompound has an excellent carrier-transport property, a light-emittingelement including the compound can be driven at a low voltage, which ispreferable.

As a benzo[a]anthracene compound of this embodiment, abenzo[a]anthracene compound having a structure in which a carbazolylgroup is bonded to a benzo[a]anthracene skeleton via a substituted orunsubstituted m-phenylene group has a wide band gap, and therefore cansuitably be used especially in a light-emitting element that emits lightwith high energy such as blue light, which is preferable.

Example 4 of Compound

In the benzo[a]anthracene compound of this embodiment, the carbazolylgroup is preferably bonded to the benzo[a]anthracene group via asubstituted or unsubstituted p-phenylene group, in which case thebenzo[a]anthracene compound can have improved stability and can besynthesized with higher purity. The above-described benzo[a]anthracenecompound is the benzo[a]anthracene compound represented by the followinggeneral formula (G4) or (G5).

In the above general formula (G4), R¹ to R¹⁰, R²¹ to R²⁸, and R⁴¹ to R⁴⁴each independently represent any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.Furthermore, R¹¹ represents any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted phenyl group. Specific examples of thealkyl group having 1 to 6 carbon atoms include a methyl group, an ethylgroup, a propyl group, an isopropyl group, a butyl group, an isobutylgroup, a tert-butyl group, an n-hexyl group, and the like. Specificexamples of a cycloalkyl group having 3 to 6 carbon atoms include acyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexylgroup, and the like. Specific examples of the aryl group having 6 to 13carbon atoms are a phenyl group, a naphthyl group, a biphenyl group, afluorenyl group, and the like. The above aryl group or phenyl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or anaryl group having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, and the like. Note that R¹, R², R³, R⁴, R⁶, R⁷, R¹, R⁹,and R¹⁰ are each preferably hydrogen in that synthesis becomes easy andthe advantage in material cost can be obtained.

In the above general formula (G5), R¹ to R¹⁰, R³¹ to R³⁵, and R⁴¹ to R⁴⁴each independently represent any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms.Furthermore, R¹¹ represents any of hydrogen, an alkyl group having 1 to6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and asubstituted or unsubstituted phenyl group. Specific examples of thealkyl group having 1 to 6 carbon atoms include a methyl group, an ethylgroup, a propyl group, an isopropyl group, a butyl group, an isobutylgroup, a tert-butyl group, an n-hexyl group, and the like. Specificexamples of a cycloalkyl group having 3 to 6 carbon atoms include acyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexylgroup, and the like. Specific examples of the aryl group having 6 to 13carbon atoms are a phenyl group, a naphthyl group, a biphenyl group, afluorenyl group, and the like. The above aryl group or phenyl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or anaryl group having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, and the like. Note that R¹, R², R³, R⁴, R⁶, R⁷, R¹, R⁹,and R¹⁰ are each preferably hydrogen in that synthesis becomes easy andthe advantage in material cost can be obtained.

Examples of Substituents

As the carbazolyl group represented by Ar in the general formula (G1),for example, groups represented by structure formulae (Cz-1) to (Cz-7)below can be used. Note that the group that can be used as A is notlimited to these.

As the arylene group represented by Ar in the above general formulae(G1) to (G3), for example, groups represented by structure formulae(Ar-1) to (Ar-18) below can be used. Note that the group that can beused as Ar is not limited to these.

For example, groups represented by structure formulae (R-1) to (R-29)below can be used as the alkyl group or aryl group represented by any ofR¹ to R¹⁰ in the above general formulae (G1) to (G5), R²¹ to R²⁸ in theabove general formulae (G2) and (G4), R³¹ to R³⁵ in the above generalformulae (G3) and (G5), and R⁴¹ to R⁴⁴ in the above general formulae(G4) and (G5). Note that the group which can be used as an alkyl groupor an aryl group are not limited thereto.

As the alkyl group or phenyl group represented by R¹¹ in the abovegeneral formulae (G1) to (G5), for example, groups represented by theabove structure formulae (R-1) to (R-22) can be used. Note that groupswhich can be used as an alkyl group or a phenyl group are not limitedthereto.

Specific Examples of Compounds

Specific examples of structures of the benzo[a]anthracene compoundsrepresented by the above general formulae (G1) to (G5) are representedby structure formulae (100) to (125) below, and the like. Note that thebenzo[a]anthracene compounds represented by the above general formulae(G1) to (G5) are not limited to the following examples.

Since a delayed fluorescence component due to TTA accounts for a largeproportion in a benzo[a]anthracene compound of this embodiment asdescribed above, the benzo[a]anthracene compound can suitably be used asa host material in a light-emitting element; a light-emitting elementwith high emission efficiency can thus be fabricated. Abenzo[a]anthracene compound of this embodiment has a wide band gap andcan suitably be used as a host material or a carrier-transport materialparticularly in a blue light-emitting element; a blue light-emittingelement with high emission efficiency can thus be fabricated. Abenzo[a]anthracene compound of this embodiment has an excellentcarrier-transport property and can suitably be used as a host materialor a carrier-transport material in a light-emitting element; alight-emitting element that can be driven at a low voltage can thus befabricated. In addition, since a benzo[a]anthracene compound of thisembodiment is highly resistant to repetition of oxidation and reduction,use of the benzo[a]anthracene compound in a light-emitting elementenables the long operating life of the light-emitting element. Asdescribed above, a benzo[a]anthracene compound of this embodiment is amaterial suitable for a light-emitting element.

A film of a benzo[a]anthracene compound of this embodiment can be formedby an evaporation method (including a vacuum evaporation method), aninkjet method, a coating method, gravure printing, or the like.

Note that the compound described in this embodiment can be used incombination with any of the structures described in the otherembodiments as appropriate.

Embodiment 3

In this embodiment, a method of synthesizing the benzo[a]anthracenecompound represented by the general formula (G1) is described. As themethod of synthesizing the benzo[a]anthracene compound, a variety ofreactions can be employed. For example, the benzo[a]anthracene compoundrepresented by the general formula (G1) can be synthesized by asynthesis reaction shown below, for example. Note that the method ofsynthesizing the benzo[a]anthracene compound which is one embodiment ofthe present invention is not limited to the following synthesis method.

The compound represented by the general formula (G1) can be synthesizedas in the synthesis scheme (A-1) below. Specifically, abenzo[a]anthracene compound (a1) and an aryl compound (a2) are coupled,whereby the compound represented by the general formula (G1) can beobtained.

In the synthesis scheme (A-1), A represents a substituted orunsubstituted carbazolyl group, and X¹ and X² each independentlyrepresent a halogen group, a trifluoromethanesulfonyl group, a boronicacid group, an organoboron group, a halogenated magnesium group, anorganotin group, or the like. In the case where X¹ represents a halogengroup or a trifluoromethanesulfonyl group, X² represents a boronic acidgroup, an organoboron group, a halogenated magnesium group, an organotingroup, or the like. In the case where X¹ represents a boronic acidgroup, an organoboron group, a halogenated magnesium group, or anorganotin group, X² represents a halogen group or atrifluoromethanesulfonyl group.

In the synthesis scheme (A-1), R¹ to R¹⁰ each independently representany of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, and a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms, R¹¹ represents any of hydrogen,an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3to 6 carbon atoms, and a substituted or unsubstituted phenyl group, andAr represents an arylene group having 6 to 13 carbon atoms. The arylenegroup may include one or more substituents and the substituents may bebonded to each other to form a ring.

When a Suzuki-Miyaura coupling reaction using a palladium catalyst isperformed in a synthesis scheme (A-1), X¹ and X² each independentlyrepresent a halogen group, a boronic acid group, an organoboron group,or a trifluoromethanesulfonyl group, and the halogen group ispreferably, iodine, bromine, or chlorine. In the reaction, a palladiumcompound such as bis(dibenzylideneacetone)palladium(0), palladium(II)acetate, [1,1′-bis(diphenylphosphino)ferrocene]palladium(II) dichloride,or tetrakis(triphenylphosphine)palladium(0) and a ligand such astri(tert-butyl)phosphine, tri(n-hexyl)phosphine, tricyclohexylphosphine,di(1-adamantyl)-n-butylphosphine,2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl, ortri(ortho-tolyl)phosphine can be used. In addition, in the reaction, anorganic base such as sodium tert-butoxide, an inorganic base such aspotassium carbonate, cesium carbonate, or sodium carbonate, or the likecan be used. In the reaction, toluene, xylene, benzene, mesitylene,tetrahydrofuran, dioxane, ethanol, methanol, water, or the like can beused as a solvent. Reagents that can be used in the reaction are notlimited thereto.

The reaction performed in the synthesis scheme (A-1) is not limited to aSuzuki-Miyaura coupling reaction, and a Migita-Kosugi-Stille couplingreaction using an organotin compound, a Kumada-Tamao-Corriu couplingreaction using a Grignard reagent, a Negishi coupling reaction using anorganozinc compound, or the like can also be employed.

In the above-described manner, the benzo[a]anthracene compoundrepresented by the general formula (G1) can be synthesized.

Note that this embodiment can be used in combination with any of thestructures described in the other embodiments as appropriate.

Embodiment 4

In this embodiment, structure examples of a light-emitting elementincluding the benzo[a]anthracene compound in Embodiment 2 are describedbelow with reference to FIG. 3 , FIG. 4 , and FIGS. 5A and 5B.

Structure Example 1 of Light-Emitting Element

In FIG. 3 , a light-emitting element 152 includes an EL layer 105between a pair of electrodes, and any of the layers in the EL layer 105includes the benzo[a]anthracene compound described in Embodiment 2.

The EL layer 105 can include the hole-injection layer 111, thehole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 which are described in Embodiment 1, inaddition to a light-emitting layer 135. The stacked layer structure ofthe EL layer 105 is not limited thereto.

The materials described in Embodiment 1 can be used for the pair ofelectrodes (electrodes 101 and 102), the hole-injection layer 111, thehole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 in this embodiment. As a guest material 137in the light-emitting layer 135, the material for the guest material 132which is described in Embodiment 1 can be used.

The benzo[a]anthracene compound described in Embodiment 2, in which adelayed fluorescence component due to TTA accounts for a largeproportion of emitted light, is suitable particularly for the hostmaterial 136 in the light-emitting element 152. When thebenzo[a]anthracene compound described in Embodiment 2 is used as thehost material 136 in the light-emitting element 152, a light-emittingelement with high emission efficiency can be fabricated. Alight-emitting element including the benzo[a]anthracene compound inwhich a delayed fluorescence component accounts for 20% or more of lightemitted from the EL layer 105 can be fabricated. Furthermore, thebenzo[a]anthracene compound having a wide band gap is suitable for ahost material or a carrier-transport material particularly in a bluelight-emitting element. Thus, the structure of this embodiment canprovide a light-emitting element with high emission efficiency and anemission spectrum peak in the blue wavelength range. Furthermore, thebenzo[a]anthracene compound having an excellent carrier-transportproperty is suitable for a host material or a carrier-transport materialin a light-emitting element. Thus, the structure of this embodiment canprovide a light-emitting element that can be driven at a low voltage.The benzo[a]anthracene compound, which is highly resistant to repetitionof oxidation and reduction, can provide a light-emitting element havinga long operating life.

Structure Example 2 of Light-Emitting Element

Next, a structure example different from the light-emitting elementillustrated in FIG. 3 is described below with reference to FIG. 4 .

FIG. 4 is a cross-sectional view illustrating a light-emitting elementof one embodiment of the present invention. In FIG. 4 , a portion havinga function similar to that in FIG. 3 is represented by the same hatchpattern as in FIG. 3 and not especially denoted by a reference numeralin some cases. In addition, common reference numerals are used forportions having similar functions, and a detailed description of theportions is omitted in some cases.

A light-emitting element 250 in FIG. 4 has a bottom-emission structurein which light is extracted through the substrate 200. However, oneembodiment of the present invention is not limited to this structure andmay have a top-emission structure in which light emitted from thelight-emitting element is extracted in the direction opposite to thesubstrate 200 or a dual-emission structure in which light emitted fromthe light-emitting element is extracted in both top and bottomdirections of the substrate 200 over which the light-emitting element isformed.

The light-emitting element 250 includes the electrode 101 and theelectrode 102 over the substrate 200. Between the electrodes 101 and102, a light-emitting layer 123B, a light-emitting layer 123G, and alight-emitting layer 123R are provided. The hole-injection layer 111,the hole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 are also provided.

In the case where the light-emitting element has a bottom emissionstructure, the electrode 101 preferably has a function of transmittinglight and the electrode 102 preferably has a function of reflectinglight.

In the light-emitting element 250 illustrated in FIG. 4 , a partition140 is present between a region 221B and a region 221G and between theregion 221G and a region 221R, in each of which the components areinterposed between the electrodes 101 and 102. The partition 140 has aninsulating property. The partition 140 covers an end portion of theelectrode 101 and has openings overlapping with the electrodes. With thepartition 140, the electrode 101 in the regions over the substrate 200can have the shape of separate islands.

The light-emitting layers 123B, 123G, and 123R preferably includelight-emitting materials having functions of emitting light of differentcolors. For example, with the light-emitting layer 123B including alight-emitting material having a function of emitting blue light, thelight-emitting layer 123G including a light-emitting material having afunction of emitting green light, and the light-emitting layer 123Rincluding a light-emitting material having a function of emitting redlight, the light-emitting element 250 can be used in a display devicecapable of full-color display. The thicknesses of the light-emittinglayers may be the same or different.

At least one of the light-emitting layers 123B, 123G, and 123Rpreferably includes the benzo[a]anthracene compound described inEmbodiment 2, in which case a light-emitting element including a regionin which the delayed fluorescence component accounts for 20% or more oflight emitted from the light-emitting layers can be fabricated. The useof the benzo[a]anthracene compound described in Embodiment 2 especiallyin the light-emitting layer 123B enables a light-emitting element withhigh emission efficiency and an emission spectrum peak in the bluewavelength range.

One or more of the light-emitting layers 123B, 123G, and 123R mayinclude two or more stacked layers.

When at least one light-emitting layer includes the benzo[a]anthracenecompound described in Embodiment 2 as described above and thelight-emitting element 250 including the light-emitting layer is used ineach sub-pixel of pixels in a display device, the display device canhave high emission efficiency. The display device including thelight-emitting element 250 can thus have reduced power consumption.

Structure Examples 3 of Light-Emitting Element

Next, structure examples different from the light-emitting elementillustrated in FIG. 3 and FIG. 4 are described below with reference toFIGS. 5A and 5B.

FIGS. 5A and 5B are cross-sectional views illustrating light-emittingelements of one embodiment of the present invention. In FIGS. 5A and 5B,a portion having a function similar to those in FIG. 3 and FIG. 4 isrepresented by the same hatch pattern as in FIG. 3 and FIG. 4 and notespecially denoted by a reference numeral in some cases. In addition,common reference numerals are used for portions having similarfunctions, and a detailed description of the portions is omitted in somecases.

FIGS. 5A and 5B each illustrate a structure example of a tandemlight-emitting element in which a plurality of light-emitting layers arestacked between a pair of electrodes with a charge-generation layer 115provided between the light-emitting layers. A light-emitting element 252illustrated in FIG. 5A has a top-emission structure in which light isextracted in a direction opposite to the substrate 200, and alight-emitting element 254 illustrated in FIG. 5B has a bottom-emissionstructure in which light is extracted to the substrate 200 side.However, one embodiment of the present invention is not limited to thesestructures and may have a dual-emission structure in which light emittedfrom the light-emitting element is extracted in both top and bottomdirections of the substrate 200 over which the light-emitting element isformed.

The light-emitting elements 252 and 254 each include the electrode 101,the electrode 102, an electrode 103, and an electrode 104 over thesubstrate 200. A light-emitting layer 160, the charge-generation layer115, and a light-emitting layer 170 are provided between the electrode101 and the electrode 102, between the electrode 102 and the electrode103, and between the electrode 102 and the electrode 104. Thehole-injection layer 111, the hole-transport layer 112, theelectron-transport layer 113, the electron-injection layer 114, thehole-injection layer 116, the hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 arefurther provided.

The electrode 101 includes a conductive layer 101 a and a conductivelayer 101 b on and in contact with the conductive layer 101 a. Theelectrode 103 includes a conductive layer 103 a and a conductive layer103 b on and in contact with the conductive layer 103 a. The electrode104 includes a conductive layer 104 a and a conductive layer 104 b onand in contact with the conductive layer 104 a.

In the light-emitting element 252 illustrated in FIG. 5A and thelight-emitting element 254 illustrated in FIG. 5B, the partition 140 ispresent between a region 222B in which the components are interposedbetween the electrodes 101 and 102 and a region 222G in which thecomponents are interposed between the electrodes 102 and 103. Thepartition 140 is present also between the region 222G and a region 222Rin which the components are interposed between the electrodes 102 and104. The partition 140 has an insulating property. The partition 140covers end portions of the electrodes 101, 103, and 104 and has openingsoverlapping with the electrodes. With the partition 140, the electrodesin the regions over the substrate 200 can have the shape of separateislands.

The light-emitting elements 252 and 254 each include a substrate 220provided with an optical element 224B, an optical element 224G, and anoptical element 224R in the direction in which light emitted from theregion 222B, light emitted from the region 222G, and light emitted fromthe region 222R are extracted. The light emitted from each region isemitted outside the light-emitting element through each optical element.In other words, the light from the region 222B, the light from theregion 222G, and the light from the region 222R are emitted through theoptical element 224B, the optical element 224G, and the optical element224R, respectively.

The optical elements 224B, 224G, and 224R each have a function ofselectively transmitting light of a particular color out of incidentlight. For example, the light emitted from the region 222B through theoptical element 224B is blue light, the light emitted from the region222G through the optical element 224G is green light, and the lightemitted from the region 222R through the optical element 224R is redlight.

Note that in FIGS. 5A and 5B, blue light (B), green light (G), and redlight (R) emitted from the regions through the optical elements areschematically illustrated by the arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. Thelight-blocking layer 223 has a function of blocking light emitted fromthe adjacent regions. Note that a structure without the light-blockinglayer 223 may also be employed.

<<Microcavity>>

Furthermore, the light-emitting elements 252 and 254 each have amicrocavity structure.

Light emitted from the light-emitting layers 160 and 170 resonatesbetween a pair of electrodes (e.g., the lower electrode 101 and theupper electrode 102). In each of the light-emitting elements 252 and254, the thicknesses of the conductive layers (the conductive layer 101b, the conductive layer 103 b, and the conductive layer 104 b) in eachregion are adjusted so that the wavelength of light emitted from thelight-emitting layers 160 and 170 can be intensified. Note that thethickness of at least one of the hole-injection layer 111 and thehole-transport layer 112 may differ between the regions so that thewavelength of light emitted from the light-emitting layers 160 and 170is intensified.

For example, in the case where the refractive index of the conductivematerial having a function of reflecting light in the electrodes 101 to104 is lower than the refractive index of the light-emitting layer 160or 170, the thickness of the conductive layer 101 b of the electrode 101is adjusted so that the optical path length between the electrode 101and the electrode 102 is m_(B)λ_(B)/² (m_(B) is a natural number andλ_(B) is a wavelength of light which is intensified in the region 222B).Similarly, the thickness of the conductive layer 103 b of the electrode103 is adjusted so that the optical path length between the electrode103 and the electrode 102 is m_(G)λG/² (m_(G) is a natural number andλ_(G) is a wavelength of light which is intensified in the region 222G).Furthermore, the thickness of the conductive layer 104 b of theelectrode 104 is adjusted so that the optical path length between theelectrode 104 and the electrode 102 is m_(R)λ_(R)/2 (m_(R) is a naturalnumber and λ_(R) is a wavelength of light which is intensified in theregion 222R).

In the above manner, with the microcavity structure, in which theoptical path length between the pair of electrodes in the respectiveregions is adjusted, scattering and absorption of light in the vicinityof the electrodes can be suppressed, resulting in high light extractionefficiency. In the above structure, each of the conductive layers 101 b,103 b, and 104 b preferably has a function of transmitting light. Thematerials of the conductive layers 101 b, 103 b, and 104 b may be thesame or different. The conductive layers 101 b, 103 b, and 104 b mayeach have two or more stacked layers.

Note that since the light-emitting element 252 illustrated in FIG. 5Ahas a top-emission structure, it is preferable that the conductive layer101 a of the electrode 101, the conductive layer 103 a of the electrode103, and the conductive layer 104 a of the electrode 104 have a functionof reflecting light. In addition, it is preferable that the electrode102 have functions of transmitting light and reflecting light.

Since the light-emitting element 254 illustrated in FIG. 5B has abottom-emission structure, it is preferable that the conductive layer101 a of the electrode 101, the conductive layer 103 a of the electrode103, and the conductive layer 104 a of the electrode 104 have functionsof transmitting light and reflecting light. In addition, it ispreferable that the electrode 102 have a function of reflecting light.

Materials used for the conductive layers 101 a, 103 a, and 104 a may bethe same or different in each of the light-emitting elements 252 and254. When the conductive layers 101 a, 103 a, and 104 a are formed usingthe same materials, manufacturing cost of the light-emitting elements252 and 254 can be reduced. The conductive layers 101 a, 103 a, and 104a may each have two or more stacked layers.

At least one of the light-emitting layers 160 and 170 preferablyincludes the benzo[a]anthracene compound described in Embodiment 2, inwhich case a light-emitting element including a region in which thedelayed fluorescence component accounts for 20% or more of light emittedfrom the light-emitting layers can be fabricated. Particularly in theregion 222B, the light-emitting element can have high emissionefficiency and an emission spectrum peak in the blue wavelength range.

The light-emitting layers 160 and 170 can each have a stacked-layerstructure of two layers, for example, a light-emitting layer 170 a and alight-emitting layer 170 b. Two kinds of light-emitting materials (afirst compound and a second compound) having functions of emitting lightof different colors are used in the two light-emitting layers, so thatlight of a plurality of emission colors can be obtained at the sametime. It is particularly preferable to select light-emitting materialsso that white light can be obtained by combining light emission from thelight-emitting layers 160 and 170.

The light-emitting layer 160 or 170 may have a structure in which threeor more layers are stacked or may include a layer containing nolight-emitting material.

When at least one light-emitting layer includes the benzo[a]anthracenecompound described in Embodiment 2 as described above and thelight-emitting element 252 or 254 including the light-emitting layer isused in each of pixels in a display device, the display device can havehigh emission efficiency. The display device including thelight-emitting element 252 or 254 can thus have reduced powerconsumption.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, light-emitting elements having structures differentfrom those described in Embodiments 1 and 4 and emission mechanisms ofthe light-emitting elements are described below with reference to FIGS.6A and 6B and FIGS. 7A and 7B.

Structure Example 1 of Light-Emitting Element

FIG. 6A is a schematic cross-sectional view of a light-emitting element450.

The light-emitting element 450 illustrated in FIG. 6A includes aplurality of light-emitting units (a light-emitting unit 441 and alight-emitting unit 442 in FIG. 6A) between a pair of electrodes (anelectrode 401 and an electrode 402). One light-emitting unit has thesame structure as the EL layer 100 illustrated in FIG. 1A or the ELlayer 105 illustrated in FIG. 3 . That is, the light-emitting element150 in FIG. 1A and the light-emitting element 152 illustrated in FIG. 3each include one light-emitting unit, while the light-emitting element450 includes a plurality of light-emitting units. Note that theelectrode 401 functions as an anode and the electrode 402 functions as acathode in the following description of the light-emitting element 450;however, the functions may be interchanged in the light-emitting element450.

In the light-emitting element 450 illustrated in FIG. 6A, thelight-emitting unit 441 and the light-emitting unit 442 are stacked, anda charge-generation layer 445 is provided between the light-emittingunit 441 and the light-emitting unit 442. Note that the light-emittingunit 441 and the light-emitting unit 442 may have the same structure ordifferent structures. For example, it is preferable that the EL layer100 illustrated in FIG. 1A or the EL layer 105 illustrated in FIG. 3 beused in the light-emitting unit 441 and that a light-emitting layercontaining a phosphorescent material as a light-emitting material beused in the light-emitting unit 442.

That is, the light-emitting element 450 includes a light-emitting layer420 and a light-emitting layer 430. The light-emitting unit 441 includesa hole-injection layer 411, a hole-transport layer 412, anelectron-transport layer 413, and an electron-injection layer 414 inaddition to the light-emitting layer 420. The light-emitting unit 442includes a hole-injection layer 416, a hole-transport layer 417, anelectron-transport layer 418, and an electron-injection layer 419 inaddition to the light-emitting layer 430.

The charge-generation layer 445 contains a composite material of anorganic compound and an acceptor substance. For the composite material,the composite material that can be used for the hole-injection layer 111described in Embodiment 1 may be used. As the organic compound, avariety of compounds such as an aromatic amine compound, a carbazolecompound, an aromatic hydrocarbon, and a high molecular compound (suchas an oligomer, a dendrimer, or a polymer) can be used. An organiccompound having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferablyused. Note that any other material may be used as long as it has aproperty of transporting more holes than electrons. Since the compositematerial of an organic compound and an acceptor substance has excellentcarrier-injection and carrier-transport properties, low-voltage drivingor low-current driving can be realized. Note that when a surface of alight-emitting unit on the anode side is in contact with thecharge-generation layer 445 as that of the light-emitting unit 442, thecharge-generation layer 445 can also serve as a hole-injection layer ora hole-transport layer of the light-emitting unit; thus, ahole-injection layer or a hole-transport layer does need not be includedin the light-emitting unit.

The charge-generation layer 445 may have a stacked-layer structure of alayer containing the composite material of an organic compound and anacceptor substance and a layer containing another material. For example,the charge-generation layer 445 may be formed using a combination of alayer containing the composite material of an organic compound and anacceptor substance with a layer containing one material selected fromamong materials having an electron donating property and a materialhaving a high electron-transport property. Furthermore, thecharge-generation layer 445 may be formed using a combination of a layercontaining the composite material of an organic compound and an acceptorsubstance with a layer including a transparent conductive film.

The charge-generation layer 445 provided between the light-emitting unit441 and the light-emitting unit 442 may have any structure as long aselectrons can be injected to the light-emitting unit on one side andholes can be injected into the light-emitting unit on the other sidewhen a voltage is applied between the electrode 401 and the electrode402. For example, in FIG. 6A, the charge-generation layer 445 injectselectrons into the light-emitting unit 441 and holes into thelight-emitting unit 442 when a voltage is applied such that thepotential of the electrode 401 is higher than that of the electrode 402.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 6A; however, a similar structure can be appliedto a light-emitting element in which three or more light-emitting unitsare stacked. With a plurality of light-emitting units partitioned by thecharge-generation layer between a pair of electrodes as in thelight-emitting element 450, it is possible to provide a light-emittingelement which can emit light with high luminance with the currentdensity kept low and has a long lifetime. A light-emitting element withlow power consumption can be provided.

When the structure of the EL layer 100 or the EL layer 105 is applied toat least one of the plurality of units, a light-emitting element withhigh emission efficiency can be provided. In particular, use of abenzo[a]anthracene compound in at least one light-emitting layer canprovide a light-emitting element with high emission efficiency.

The light-emitting layer 420 includes a host material 421 and a guestmaterial 422. The light-emitting layer 430 includes a host material 431and a guest material 432. The host material 431 includes an organiccompound 431_1 and an organic compound 431_2.

In this embodiment, the light-emitting layer 420 has a structure similarto that of the light-emitting layer 130 in FIG. 1A or the light-emittinglayer 135 illustrated in FIG. 3 . That is, in the light-emitting layer420, the host material 421 and the guest material 422 correspond to thehost material 131 and the guest material 132, respectively, in thelight-emitting layer 130, or alternatively correspond to the hostmaterial 136 and the guest material 137, respectively, in thelight-emitting layer 135. In the following description, the guestmaterial 432 included in the light-emitting layer 430 is aphosphorescent material. Note that the electrode 401, the electrode 402,the hole-injection layers 411 and 416, the hole-transport layers 412 and417, the electron-transport layers 413 and 418, and theelectron-injection layers 414 and 419 have functions similar to those ofthe electrode 101, the electrode 102, the hole-injection layer 111, thehole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 in Embodiment 1, respectively. Therefore,detailed description thereof is omitted in this embodiment.

<Emission Mechanism of Light-Emitting Layer 420>

An emission mechanism of the light-emitting layer 420 is similar to thatof the light-emitting layer 130 in FIG. 1A or the EL layer 135 in FIG. 3.

<Emission Mechanism of Light-Emitting Layer 430>

Next, an emission mechanism of the light-emitting layer 430 isdescribed.

The organic compound 431_1 and the organic compound 431_2 which areincluded in the light-emitting layer 430 form an exciplex. The organiccompound 431_1 serves as a host material and the organic compound 431_2serves as an assist material in the description here.

Although it is acceptable as long as the combination of the organiccompound 431_1 and the organic compound 431_2 can form an exciplex inthe light-emitting layer 430, it is preferred that one organic compoundbe a material having a hole-transport property and the other organiccompound be a material having an electron-transport property.

FIG. 6B illustrates the correlation of energy levels of the organiccompound 4311, the organic compound 431_2, and the guest material 432 inthe light-emitting layer 430. What terms and signs in FIG. 6B are listedbelow.

Host (431_1): the organic compound 431_1 (host material)

Assist (431_2): the organic compound 431_2 (assist material)

Guest (432): the guest material 432 (phosphorescent material)

Exciplex: exciplex

S_(PH): the level of the lowest singlet excited state of the organiccompound 431_1

T_(PH): the level of the lowest triplet excited state of the organiccompound 431_1

T_(PG): the level of the lowest triplet excited state of the guestmaterial 432 (phosphorescent material)

S_(E): the level of the lowest singlet excited state of the exciplex

T_(E): the level of the lowest triplet excited state of the exciplex

The level (S_(E)) of the lowest singlet excited state of the exciplexformed by the organic compounds 431_1 and 431_2 and the level (T_(E)) ofthe lowest triplet excited state of the exciplex are close to each other(see Route C in FIG. 6B).

Both energies of S_(E) and T_(E) of the exciplex are then transferred tothe level of the lowest triplet excited state of the guest material 432(phosphorescent material); thus, light emission is obtained (see Route Din FIG. 6B).

The above-described processes through Route C and Route D may bereferred to as exciplex-triplet energy transfer (ExTET) in thisspecification and the like.

When one of the organic compounds 431_1 and 431_2 receiving holes andthe other receiving electrons come close to each other, the exciplex isformed at once. Alternatively, when one compound is brought into anexcited state, the one immediately interacts with the other compound toform the exciplex. Therefore, most excitons in the light-emitting layer430 exist as exciplexes. The band gap of the exciplex is narrower thanthat of each of the organic compounds 431_1 and 431_2; therefore, thedrive voltage can be lowered when the exciplex is formed byrecombination of a hole and an electron.

When the light-emitting layer 430 has the above structure, lightemission from the guest material 432 (phosphorescent material) of thelight-emitting layer 430 can be efficiently obtained.

Note that light emitted from the light-emitting layer 420 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 430. Since the luminance of a light-emittingelement using a phosphorescent material emitting light with a shortwavelength tends to be degraded quickly, fluorescence with a shortwavelength is employed so that a light-emitting element with lessdegradation of luminance can be provided.

Furthermore, the light-emitting layer 420 and the light-emitting layer430 may be made to emit light with different emission wavelengths, sothat the light-emitting element can be a multicolor light-emittingelement. In that case, the emission spectrum of the light-emittingelement is formed by combining light having different emission peaks,and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission.When the light-emitting layer 420 and the light-emitting layer 430 emitlight of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering propertythat is formed of three primary colors or four or more colors can beobtained by using a plurality of light-emitting materials emitting lightwith different wavelengths for one of the light-emitting layers 420 and430 or both. In that case, one of the light-emitting layers 420 and 430or both may be divided into layers and each of the divided layers maycontain a light-emitting material different from the others.

<Material that can be Used in Light-Emitting Layers>

Next, materials that can be used in the light-emitting layers 420 and430 are described.

<<Material that can be used in light-emitting layer 420>

As a material that can be used in the light-emitting layer 420, amaterial that can be used in the light-emitting layer 130 in Embodiment1 or the light-emitting layer 135 in Embodiment 4 may be used.

<<Material that can be Used in Light-Emitting Layer 430>>

In the light-emitting layer 430, the organic compound 431_1 (hostmaterial) exists in the highest proportion in weight ratio, and theguest material 432 (phosphorescent material) is dispersed in the organiccompound 431_1 (host material).

Examples of the organic compound 431_1 (host material) include a zinc-or aluminum-based metal complex, an oxadiazole derivative, a triazolederivative, a benzimidazole derivative, a quinoxaline derivative, adibenzoquinoxaline derivative, a dibenzothiophene derivative, adibenzofuran derivative, a pyrimidine derivative, a triazine derivative,a pyridine derivative, a bipyridine derivative, a phenanthrolinederivative, and the like. Other examples are an aromatic amine, acarbazole derivative, and the like.

As the guest material 432 (phosphorescent material), an iridium-,rhodium-, or platinum-based organometallic complex or metal complex canbe used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazineligand, an isoquinoline ligand, and the like can be given. As the metalcomplex, a platinum complex having a porphyrin ligand and the like canbe given.

As the organic compound 431_2 (assist material), a substance which canform an exciplex together with the organic compound 431_1 is used. Inthat case, it is preferable that the organic compound 4311, the organiccompound 431_2, and the guest material 432 (phosphorescent material) beselected such that the emission peak of the exciplex overlaps with anadsorption band, specifically an adsorption band on the longestwavelength side, of a triplet metal to ligand charge transfer (MLCT)transition of the guest material 432 (phosphorescent material). Thismakes it possible to provide a light-emitting element with drasticallyimproved emission efficiency. Note that in the case where a thermallyactivated delayed fluorescent material is used instead of thephosphorescent material, it is preferable that the adsorption band onthe longest wavelength side be a singlet absorption band.

As the light-emitting material contained in the light-emitting layer430, any material can be used as long as the material can converttriplet excited energy into light emission. As an example of thematerial that can convert triplet excited energy into light emission, athermally activated delayed fluorescent (TADF) material can be given inaddition to the phosphorescent material. Therefore, the term“phosphorescent material” in the description can be replaced with theterm “thermally activated delayed fluorescent material”. Note that thethermally activated delayed fluorescent material is a material that canup-convert a triplet excited state into a singlet excited state (i.e.,reverse intersystem crossing is possible) using a little thermal energyand efficiently exhibits light emission (fluorescence) from the singletexcited state. Thermally activated delayed fluorescence is efficientlyobtained under the condition where the difference between the tripletexcited energy level and the singlet excited energy level is preferablylarger than 0 eV and smaller than or equal to 0.2 eV, further preferablylarger than 0 eV and smaller than or equal to 0.1 eV

The material that emits thermally activated delayed fluorescence may bea material that can form a singlet excited state by itself from atriplet excited state by reverse intersystem crossing or may be acombination of two kinds of materials which form an exciplex.

There is no limitation on the emission colors of the light-emittingmaterial included in the light-emitting layer 420 and the light-emittingmaterial included in the light-emitting layer 430, and they may be thesame or different. Light emitted from the light-emitting materials ismixed and extracted out of the element; therefore, for example, in thecase where their emission colors are complementary colors, thelight-emitting element can emit white light. In consideration of thereliability of the light-emitting element, the emission peak wavelengthof the light-emitting material included in the light-emitting layer 420is preferably shorter than that of the light-emitting material includedin the light-emitting layer 430.

Structure Example 2 of Light-Emitting Element

Next, a structure example different from the light-emitting elementillustrated in FIGS. 6A and 6B is described below with reference toFIGS. 7A and 7B.

FIG. 7A is a schematic cross-sectional view of a light-emitting element452.

In the light-emitting element 452 illustrated in FIG. 7A, an EL layer400 is provided between a pair of electrodes (an electrode 401 and anelectrode 402). Note that in the light-emitting element 452, theelectrode 401 functions as an anode, and the electrode 402 functions asa cathode.

The EL layer 400 includes the light-emitting layers 420 and 430. As theEL layer 400 in the light-emitting element 452, the light-emittinglayers 420 and 430, the hole-injection layer 411, the hole-transportlayer 412, the electron-transport layer 418, and the electron-injectionlayer 419 are illustrated. However, this stacked-layer structure is anexample, and the structure of the EL layer 400 in the light-emittingelement 452 is not limited thereto. For example, the stacking order ofthe above layers of the EL layer 400 may be changed. Alternatively, inthe EL layer 400, another functional layer other than the above layersmay be provided. The functional layer may have a function of injecting acarrier (an electron or a hole), a function of transporting a carrier, afunction of inhibiting a carrier, or a function of generating a carrier,for example.

The light-emitting layer 420 includes the host material 421 and theguest material 422. The light-emitting layer 430 includes the hostmaterial 431 and the guest material 432. The host material 431 includesthe organic compound 431_1 and the organic compound 431_2. In thefollowing description, the guest material 422 is a fluorescent materialand the guest material 432 is a phosphorescent material.

<<Emission Mechanism of Light-Emitting Layer 420>>

The emission mechanism of the light-emitting layer 420 is similar tothat of the light-emitting layer 130 in FIG. 1A or the EL layer 135 inFIG. 3 .

<<Emission Mechanism of Light-Emitting Layer 430>>

The emission mechanism of the light-emitting layer 430 is similar tothat of the light-emitting layer 430 in FIG. 6A.

<Emission Mechanism of Light-Emitting Layers 420 and 430>

Each emission mechanism of the light-emitting layers 420 and 430 isdescribed above. As in the light-emitting element 452, in the case wherethe light-emitting layers 420 and 430 are in contact with each other,even when energy is transferred from the exciplex to the host material421 of the light-emitting layer 420 (in particular, when energy of thetriplet excited level is transferred) at an interface between thelight-emitting layer 420 and the light-emitting layer 430, tripletexcited energy can be converted into light emission in thelight-emitting layer 420.

The T₁ level of the host material 421 of the light-emitting layer 420 ispreferably lower than T₁ levels of the organic compounds 431_1 and 431_2in the light-emitting layer 430. In the light-emitting layer 420, the S₁level of the host material 421 is preferably higher than the S₁ level ofthe guest material 422 (fluorescent material) while the T₁ level of thehost material 421 is preferably lower than the T₁ level of the guestmaterial 422 (fluorescent material).

FIG. 7B shows the correlation of energy levels in the case where TTA isutilized in the light-emitting layer 420 and ExTET is utilized in thelight-emitting layer 430. What terms and signs in FIG. 7B are listedbelow.

Fluorescence EML (420): the fluorescent light-emitting layer(light-emitting layer 420)

Phosphorescence EML (430): the phosphorescent light-emitting layer(light-emitting layer 430)

S_(FH): the level of the lowest singlet excited state of the hostmaterial 421

T_(FH): the level of the lowest triplet excited state of the hostmaterial 421

S_(FG): the level of the lowest singlet excited state of the guestmaterial 422 (fluorescent material)

T_(FG): the level of the lowest triplet excited state of the guestmaterial 422 (fluorescent material)

S_(PH): the level of the lowest singlet excited state of the hostmaterial (organic compound 431_1)

T_(PH): the level of the lowest triplet excited state of the hostmaterial (organic compound 431_1)

T_(PG): the level of the lowest triplet excited state of the guestmaterial 432 (phosphorescent material)

S_(E): the level of the lowest singlet excited state of the exciplexT_(E): the level of the lowest triplet excited state of the exciplex

As shown in FIG. 7B, the exciplex exists only in an excited state; thus,exciton diffusion between the exciplexes is less likely to occur. Inaddition, because the excited levels (S_(E) and T_(E)) of the exciplexare lower than the excited levels (S_(PH) and T_(PH)) of the organiccompound 431_1 (the host material of the phosphorescent material) of thelight-emitting layer 430, energy diffusion from the exciplex to theorganic compound 431_1 does not occur. Similarly, energy diffusion fromthe exciplex to the organic compound 431_2 does not occur. That is,emission efficiency of the phosphorescent light-emitting layer(light-emitting layer 430) can be maintained because an excitondiffusion distance of the exciplex is short in the phosphorescentlight-emitting layer (light-emitting layer 430). In addition, even whenpart of the triplet excited energy of the exciplex of the phosphorescentlight-emitting layer (light-emitting layer 430) diffuses into thefluorescent light-emitting layer (light-emitting layer 420) through theinterface between the fluorescent light-emitting layer (light-emittinglayer 420) and the phosphorescent light-emitting layer (light-emittinglayer 430), energy loss can be reduced because the triplet excitedenergy in the fluorescent light-emitting layer (light-emitting layer420) caused by the diffusion is used for light emission through TTA.

The light-emitting element 452 can have high emission efficiency becauseExTET is utilized in the light-emitting layer 430 and TTA is utilized inthe light-emitting layer 420 as described above so that energy loss isreduced. As in the light-emitting element 452, in the case where thelight-emitting layer 420 and the light-emitting layer 430 are in contactwith each other, the number of EL layers 400 as well as the energy losscan be reduced. Therefore, a light-emitting element with lowmanufacturing cost can be obtained.

Note that the light-emitting layer 420 and the light-emitting layer 430need not be in contact with each other. In that case, it is possible toprevent energy transfer by the Dexter mechanism (particularly tripletenergy transfer) from the organic compound 431_1 or 431_2 in an excitedstate or the guest material 432 (phosphorescent material) in an excitedstate which is generated in the light-emitting layer 430 to the hostmaterial 421 or the guest material 422 (fluorescent material) in thelight-emitting layer 420. Therefore, the thickness of a layer providedbetween the light-emitting layer 420 and the light-emitting layer 430may be several nanometers.

The layer provided between the light-emitting layer 420 and thelight-emitting layer 430 may contain a single material or both ahole-transport material and an electron-transport material. In the caseof a single material, a bipolar material may be used. The bipolarmaterial here refers to a material in which the ratio between theelectron mobility and the hole mobility is 100 or less. Alternatively,the hole-transport material, the electron-transport material, or thelike may be used. At least one of materials included in the layer may bethe same as the host material (organic compound 431_1 or 431_2) of thelight-emitting layer 430. This facilitates the manufacture of thelight-emitting element and reduces the drive voltage. Furthermore, thehole-transport material and the electron-transport material may form anexciplex, which effectively prevents exciton diffusion. Specifically, itis possible to prevent energy transfer from the host material (organiccompound 431_1 or 431_2) in an excited state or the guest material 432(phosphorescent material) in an excited state of the light-emittinglayer 430 to the host material 421 or the guest material 422(fluorescent material) in the light-emitting layer 420.

Note that in the light-emitting element 452, a carrier recombinationregion is preferably distributed to some extent. Therefore, it ispreferable that the light-emitting layer 420 or 430 have an appropriatedegree of carrier-trapping property. It is particularly preferable thatthe guest material 432 (phosphorescent material) in the light-emittinglayer 430 have an electron-trapping property. Alternatively, the guestmaterial 422 (fluorescent material) in the light-emitting layer 420preferably has a hole-trapping property.

Note that light emitted from the light-emitting layer 420 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 430. Since the luminance of a light-emittingelement using a phosphorescent material emitting light with a shortwavelength tends to be degraded quickly, fluorescence with a shortwavelength is employed so that a light-emitting element with lessdegradation of luminance can be provided.

Furthermore, the light-emitting layers 420 and 430 are made to emitlight with different emission wavelengths, so that the light-emittingelement can be a multicolor light-emitting element. In that case, theemission spectrum is formed by combining light having different emissionpeaks, and thus has at least two peaks.

The above-described structure is suitable for obtaining white lightemission. When the light-emitting layers 420 and 430 emit light ofcomplementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering propertythat is formed of three primary colors or four or more colors can beobtained by using a plurality of light-emitting materials emitting lightwith different wavelengths for the light-emitting layer 420. In thatcase, the light-emitting layer 420 may be divided into layers and eachof the divided layers may contain a light-emitting material differentfrom the others.

<Material that can be Used in Light-Emitting Layers>

Next, materials that can be used in the light-emitting layers 420 and430 are described.

<<Material that can be Used in Light-Emitting Layer 420>>

In the light-emitting layer 420, the host material 421 is present in thehighest proportion by weight, and the guest material 422 (fluorescentmaterial) is dispersed in the host material 421. The S₁ level of thehost material 421 is preferably higher than the S₁ level of the guestmaterial 422 (fluorescent material) while the T₁ level of the hostmaterial 421 is preferably lower than the T₁ level of the guest material422 (fluorescent material).

As the host material 421, a benzo[a]anthracene compound is preferablyused to fabricate a light-emitting element with high emission efficiencyin which delayed fluorescence accounts for a large proportion of emittedlight. Specifically, the compound described in Embodiment 1 or 2 can beused.

<Materials that can be Used in Light-Emitting Layer 430>

In the light-emitting layer 430, the host material (organic compound431_1 or 431_2) is present in the highest proportion in mass ratio, andthe guest material 432 (phosphorescent material) is dispersed in thehost materials (organic compounds 431_1 and 431_2). The T₁ levels of thehost materials (organic compounds 431_1 and 431_2) of the light-emittinglayer 430 is preferably higher than the T₁ level of the guest material422 (fluorescent material) of the light-emitting layer 420.

As the host materials (organic compounds 431_1 and 431_2) and the guestmaterial 432 (phosphorescent material), those in the light-emittingelement 450 described with reference to FIGS. 6A and 6B can be used.

Note that the light-emitting layers 420 and 430 can be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, gravure printing, or the like.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment is described an example of a mode where abenzo[a]anthracene compound described in Embodiment 2 is used in anactive layer of a vertical transistor (static induction transistor:SIT), which is a kind of an organic semiconductor element.

The element has a structure in which a thin-film active layer 330including the benzo[a]anthracene compound described in Embodiment 2 isprovided between a source electrode 301 and a drain electrode 302, andgate electrodes 303 are embedded in the active layer 330, as illustratedin FIG. 8 . The gate electrode 303 is electrically connected to a meansfor applying a gate voltage, and the source electrode 301 and the drainelectrode 302 are electrically connected to a means for controlling avoltage between the source electrode and the drain electrode.

In such an element structure, when a voltage is applied between thesource electrode and the drain electrode without applying a voltage tothe gate electrode 303, a current flows (on state). Then, by applicationof a voltage to the gate electrode 303 in that state, a depletion layeris formed in the periphery of the gate electrode 303, and the currentceases flowing (off state). With such a mechanism, an organicsemiconductor element 300 operates as a transistor.

Like a light-emitting element, a vertical transistor should contain amaterial that can achieve both a high carrier-transport property andhigh film quality for an active layer; the benzo[a]anthracene compounddescribed in Embodiment 2 meets such a requirement and therefore can besuitably used.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 7

In this embodiment, a display device including a light-emitting deviceof one embodiment of the present invention is described with referenceto FIGS. 9A and 9B.

FIG. 9A is a block diagram illustrating the display device of oneembodiment of the present invention, and FIG. 9B is a circuit diagramillustrating a pixel circuit of the display device of one embodiment ofthe present invention.

<Display Device>

The display device illustrated in FIG. 9A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 802), a circuit portion provided outside the pixel portion802 and including a circuit for driving the pixels (hereinafter theportion is referred to as a driver circuit portion 804), circuits havinga function of protecting elements (hereinafter the circuits are referredto as protection circuits 806), and a terminal portion 807. Note thatthe protection circuits 806 are not necessarily provided.

A part or the whole of the driver circuit portion 804 is preferablyformed over a substrate over which the pixel portion 802 is formed.Thus, the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 804 isnot formed over the substrate over which the pixel portion 802 isformed, the part or the whole of the driver circuit portion 804 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 802 includes circuits for driving a plurality ofdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter, suchcircuits are referred to as pixel circuits 801). The driver circuitportion 804 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter the circuit isreferred to as a scan line driver circuit 804 a) and a circuit forsupplying a signal (data signal) to drive a display element in a pixel(hereinafter, the circuit is referred to as a signal line driver circuit804 b).

The scan line driver circuit 804 a includes a shift register or thelike. The scan line driver circuit 804 a receives a signal for drivingthe shift register through the terminal portion 807 and outputs asignal. For example, the scan line driver circuit 804 a receives a startpulse signal, a clock signal, or the like and outputs a pulse signal.The scan line driver circuit 804 a has a function of controlling thepotentials of wirings supplied with scan signals (hereinafter, suchwirings are referred to as scan lines GL_1 to GL_X). Note that aplurality of scan line driver circuits 804 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the scan linedriver circuit 804 a has a function of supplying an initializationsignal. Not limited thereto, the scan line driver circuit 804 a cansupply another signal.

The signal line driver circuit 804 b includes a shift register or thelike. The signal line driver circuit 804 b receives a signal (videosignal) from which a data signal is derived, as well as a signal fordriving the shift register, through the terminal portion 807. The signalline driver circuit 804 b has a function of generating a data signal tobe written in the pixel circuits 801 based on the video signal. Inaddition, the signal line driver circuit 804 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the signal line driver circuit 804 b has a function ofcontrolling the potentials of wirings supplied with data signals(hereinafter, such wirings are referred to as data lines DL_1 to DL_Y).Alternatively, the signal line driver circuit 804 b has a function ofsupplying an initialization signal. Not limited thereto, the signal linedriver circuit 804 b can supply another signal.

Alternatively, the signal line driver circuit 804 b is formed using aplurality of analog switches or the like, for example. The signal linedriver circuit 804 b can output, as the data signals, signals obtainedby time-dividing the video signal by sequentially turning on theplurality of analog switches. The signal line driver circuit 804 b mayinclude a shift register or the like.

A pulse signal and a data signal are input, through one of the pluralityof scan lines GL supplied with scan signals and one of the plurality ofdata lines DL supplied with data signals, respectively, to each of theplurality of the pixel circuits 801. Writing and holding of the datasignal in each of the plurality of pixel circuits 801 are controlled bythe scan line driver circuit 804 a. For example, to the pixel circuit801 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the scan line driver circuit 804 athrough the scan line GL_m, and a data signal is input from the signalline driver circuit 804 b through the data line DL_n in accordance withthe potential of the scan line GL_m.

The protection circuit 806 shown in FIG. 9A is connected to, forexample, the scan line GL between the scan line driver circuit 804 a andthe pixel circuits 801. Alternatively, the protection circuit 806 isconnected to the data line DL between the signal line driver circuit 804b and the pixel circuit 801. Alternatively, the protection circuit 806can be connected to a wiring between the scan line driver circuit 804 aand the terminal portion 807. Alternatively, the protection circuit 806can be electrically connected to a wiring between the signal line drivercircuit 804 b and the terminal portion 807. Note that the terminalportion 807 means a portion having terminals for inputting power,control signals, and video signals to the display device from externalcircuits.

The protection circuit 806 is a circuit which electrically conducts awiring connected to the protection circuit to another wiring when apotential out of a certain range is supplied to the wiring connected tothe protection circuit.

As illustrated in FIG. 9A, the protection circuits 806 are provided forthe pixel portion 802 and the driver circuit portion 804, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 806 is not limited to that, andfor example, the protection circuit 806 may be configured to beconnected to the scan line driver circuit 804 a or the protectioncircuit 806 may be configured to be connected to the signal line drivercircuit 804 b. Alternatively, the protection circuit 806 may beconfigured to be connected to the terminal portion 807.

In FIG. 9A, an example in which the driver circuit portion 804 includesthe scan line driver circuit 804 a and the signal line driver circuit804 b is shown; however, the structure is not limited thereto. Forexample, only the scan line driver circuit 804 a may be formed and aseparately prepared substrate where a signal line driver circuit isformed (e.g., a driver circuit substrate formed with a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted.

Structure Example of Pixel Circuit

Each of the plurality of pixel circuits 801 in FIG. 9A can have thestructure illustrated in FIG. 9B, for example.

The pixel circuit 801 shown in FIG. 9B includes transistors 852 and 854,a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 iselectrically connected to a wiring to which a data signal is supplied (adata line DL_n). A gate electrode of the transistor 852 is electricallyconnected to a wiring to which a gate signal is supplied (a scan lineGL_m).

The transistor 852 has a function of controlling whether to write a datasignal.

One of a pair of electrodes of the capacitor 862 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 854 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 854 is electrically connected to theother of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element 872 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 854.

As the light-emitting element 872, any of the light-emitting elementsdescribed in Embodiments 1 to 5 can be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 801 inFIG. 9B, the pixel circuits 801 are sequentially selected row by row bythe scan line driver circuit 804 a illustrated in FIG. 9A, whereby thetransistor 852 is turned on and a data signal is written.

When the transistor 852 is turned off, the pixel circuits 801 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 854 is controlled in accordance with thepotential of the written data signal. The light-emitting element 872emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

A light-emitting element of one embodiment of the present invention canbe used for an active matrix method in which an active element isincluded in a pixel of a display device or a passive matrix method inwhich an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements) can be used. For example, a metal insulator metal(MIM) or a thin film diode (TFD) can also be used. Since these elementscan be formed with a smaller number of manufacturing steps,manufacturing costs can be reduced or yield can be improved.Alternatively, since the size of the element is small, the apertureratio can be improved, leading to lower power consumption or higherluminance.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcosts can be reduced or yield can be improved. Alternatively, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, leading to lower power consumption, higher luminance,or the like.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 8

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention and an electronic device inwhich the display device is provided with an input device are describedwith reference to FIGS. 10A and 10B, FIGS. 11A to 11C, FIGS. 12A and12B, FIGS. 13A and 13B, and FIG. 14 .

<Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device is described as an example of an electronic device. Inaddition, an example in which a touch sensor is used as an input deviceis described.

FIGS. 10A and 10B are perspective views of the touch panel 2000. Notethat FIGS. 10A and 10B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 10B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 10B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used.Examples of the capacitive touch sensor are a surface capacitive touchsensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self-capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of a mutual capacitive type is preferablebecause multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 10B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 10A and 10B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

<Display Panel>

Next, the display device 2501 is described in detail with reference toFIG. 11A. FIG. 11A corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 10B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

In the following description, an example of using a light-emittingelement that emits white light as a display element will be described;however, the display element is not limited to such an element. Forexample, light-emitting elements that emit light of different colors maybe included so that the light of different colors can be emitted fromadjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability lower than or equal to 1×10⁻⁵g·m⁻²·day⁻¹, preferably lower than or equal to 1×10⁻⁶ g·m⁻²·day⁻¹ can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emittingelement, a flexible substrate 2510 b, and an adhesive layer 2510 c forattaching the insulating layer 2510 a and the flexible substrate 2510 bto each other. The substrate 2570 is a stacked body including aninsulating layer 2570 a for preventing impurity diffusion into thelight-emitting element, a flexible substrate 2570 b, and an adhesivelayer 2570 c for attaching the insulating layer 2570 a and the flexiblesubstrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, or an acrylic resin, polyurethane, or an epoxyresin, or a material which includes a resin having a siloxane bond suchas silicone can be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 11A, the sealing layer2560 can also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element 2550R can beprovided in a region surrounded by the substrate 2510, the substrate2570, the sealing layer 2560, and the sealant. Note that an inert gas(such as nitrogen or argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorbmoisture or the like. An ultraviolet curable resin or a heat curableresin may be used; for example, a polyvinyl chloride (PVC) based resin,an acrylic resin, a polyimide-based resin, an epoxy-based resin, asilicone-based resin, a polyvinyl butyral (PVB) based resin, or anethylene vinyl acetate (EVA) based resin can be used. For example, anepoxy-based resin or a glass frit is preferably used as the sealant. Asa material used for the sealant, a material which is impermeable tomoisture or oxygen is preferably used.

The display device 2501 includes a pixel 2502R. The pixel 2502R includesa light-emitting module 2580R.

The pixel 2502R includes the light-emitting element 2550R and atransistor 2502 t that can supply electric power to the light-emittingelement 2550R. Note that the transistor 2502 t functions as part of thepixel circuit. The light-emitting module 2580R includes thelight-emitting element 2550R and a coloring layer 2567R.

The light-emitting element 2550R includes a lower electrode, an upperelectrode, and an EL layer between the lower electrode and the upperelectrode. As the light-emitting element 2550R, any of thelight-emitting elements described in Embodiments 1 to 5 can be used, forexample.

A microcavity structure may be employed between the lower electrode andthe upper electrode so as to increase the intensity of light having aspecific wavelength.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with thelight-emitting element 2550R and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. Accordingly, part of light emitted fromthe light-emitting element 2550R passes through the coloring layer 2567Rand is emitted to the outside of the light-emitting module 2580R asindicated by an arrow in the figure.

The display device 2501 includes a light-blocking layer 2567BM on thelight extraction side. The light-blocking layer 2567BM is provided so asto surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength range, a colorfilter for transmitting light in a green wavelength range, a colorfilter for transmitting light in a blue wavelength range, a color filterfor transmitting light in a yellow wavelength range, or the like can beused. Each color filter can be formed with any of various materials by aprinting method, an inkjet method, an etching method using aphotolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer 2521 covers the transistor 2502 t. Note that theinsulating layer 2521 has a function of covering unevenness caused bythe pixel circuit. The insulating layer 2521 may have a function ofsuppressing impurity diffusion. This can prevent the reliability of thetransistor 2502 t or the like from being lowered by impurity diffusion.

The light-emitting element 2550R is formed over the insulating layer2521. A partition 2528 is provided so as to overlap with an end portionof the lower electrode of the light-emitting element 2550R. Note that aspacer for controlling the distance between the substrate 2510 and thesubstrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2503 g(1) includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a printed wiring board (PWB).

In the display device 2501, transistors with any of a variety ofstructures can be used. FIG. 11A illustrates an example of usingbottom-gate transistors; however, the present invention is not limitedto this example, and top-gate transistors may be used in the displaydevice 2501 as illustrated in FIG. 11B.

In addition, there is no particular limitation on the polarity of thetransistor 2502 t and the transistor 2503 t. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors 2502 t and 2503 t. Forexample, an amorphous semiconductor film or a crystalline semiconductorfilm may be used. Examples of semiconductor materials include Group 13semiconductors (e.g., a semiconductor including gallium), Group 14semiconductors (e.g., a semiconductor including silicon), compoundsemiconductors (including oxide semiconductors), organic semiconductors,and the like. An oxide semiconductor that has an energy gap of 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more ispreferably used for one of the transistors 2502 t and 2503 t or both, sothat the off-state current of the transistors can be reduced. Examplesof the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (Mrepresents aluminum (Al), gallium (Ga), 10 yttrium (Y), zirconium (Zr),lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)),and the like.

<Touch Sensor>

Next, the touch sensor 2595 is described in detail with reference toFIG. 11C. FIG. 11C corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 10B.

The touch sensor 2595 includes the electrodes 2591 and 2592 provided ina staggered arrangement on the substrate 2590, an insulating layer 2593covering the electrodes 2591 and 2592, and the wiring 2594 thatelectrically connects the adjacent electrodes 2591 to each other.

The electrodes 2591 and 2592 are formed using a light-transmittingconductive material. As a light-transmitting conductive material, aconductive oxide such as indium oxide, indium tin oxide, indium zincoxide, zinc oxide, or zinc oxide to which gallium is added can be used.Note that a film including graphene may be used as well. The filmincluding graphene can be formed, for example, by reducing a filmincluding graphene oxide. As a reducing method, a method withapplication of heat or the like can be employed.

The electrodes 2591 and 2592 may be formed by, for example, depositing alight-transmitting conductive material on the substrate 2590 by asputtering method and then removing an unnecessary portion by any ofvarious pattern forming techniques such as photolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond, andan inorganic insulating material such as silicon oxide, siliconoxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle greater than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<Description 2 of Touch Panel>

Next, the touch panel 2000 is described in detail with reference to FIG.12A. FIG. 12A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 10A.

In the touch panel 2000 illustrated in FIG. 12A, the display device 2501described with reference to FIG. 11A and the touch sensor 2595 describedwith reference to FIG. 11C are attached to each other.

The touch panel 2000 illustrated in FIG. 12A includes an adhesive layer2597 and an anti-reflective layer 2567 p in addition to the componentsdescribed with reference to FIGS. 11A and 11C.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, anurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2567 p is positioned in a region overlappingwith pixels. As the anti-reflective layer 2567 p, a circularlypolarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 12A is described with reference to FIG. 12B.

FIG. 12B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 12B differs from the touch panel 2000illustrated in FIG. 12A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. The light-emitting element 2550Rillustrated in FIG. 12B emits light to the side where the transistor2502 t is provided. Accordingly, part of light emitted from thelight-emitting element 2550R passes through the coloring layer 2567R andis emitted to the outside of the light-emitting module 2580R asindicated by the arrow in the figure.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 12A or 12B, light may be emitted from thelight-emitting element to one of upper and lower sides, or both, of thesubstrate.

<Method of Driving Touch Panel>

Next, an example of a method of driving a touch panel is described withreference to FIGS. 13A and 13B.

FIG. 13A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 13A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 13A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 13A also illustratescapacitors 2603 that are each formed in a region in which the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and the electrodes 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and the electrodes 2622 of the capacitor2603. When the electric field between the electrodes is shielded, forexample, a change occurs in the capacitor 2603 (mutual capacitance). Theapproach or contact of a sensing target can be sensed by utilizing thischange.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 13B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 13A. In FIG. 13B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 13B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change in accordance with changes inthe voltages of the wirings X1 to X6. The current value is decreased atthe point of approach or contact of a sensing target and accordingly thewaveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Sensor Circuit>

Although FIG. 13A illustrates a passive matrix touch sensor in whichonly the capacitor 2603 is provided at the intersection of wirings as atouch sensor, an active matrix touch sensor including a transistor and acapacitor may be used. FIG. 14 illustrates an example of a sensorcircuit included in an active matrix touch sensor.

The sensor circuit in FIG. 14 includes the capacitor 2603 and atransistor 2611, a transistor 2612, and a transistor 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 14 is described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to the node n connected to the gate of the transistor 2611.Then, a potential for turning off the transistor 2613 is applied as thesignal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613 so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 9

In this embodiment, a display module and electronic devices including alight-emitting element of one embodiment of the present invention aredescribed with reference to FIG. 15 and FIGS. 16A to 16G.

<Display Module>

In a display module 8000 in FIG. 15 , a touch sensor 8004 connected toan FPC 8003, a display device 8006 connected to an FPC 8005, a frame8009, a printed board 8010, and a battery 8011 are provided between anupper cover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present inventioncan be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchsensor 8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitivetouch sensor and may be formed to overlap with the display device 8006.A counter substrate (sealing substrate) of the display device 8006 canhave a touch sensor function. A photosensor may be provided in eachpixel of the display device 8006 so that an optical touch sensor isobtained.

The frame 8009 protects the display device 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Electronic Devices>

FIGS. 16A to 16G illustrate electronic devices. These electronic devicescan include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring or sensing force, displacement, position, speed, acceleration,angular velocity, rotational frequency, distance, light, liquid,magnetism, temperature, chemical substance, sound, time, hardness,electric field, current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared ray), a microphone9008, and the like.

The electronic devices illustrated in FIGS. 16A to 16G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions that can be provided for theelectronic devices illustrated in FIGS. 16A to 16G are not limited tothose described above, and the electronic devices can have a variety offunctions. Although not illustrated in FIGS. 16A to 16G, the electronicdevices may include a plurality of display portions. The electronicdevices may have a camera or the like and a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices illustrated in FIGS. 16A to 16G is described indetail below.

FIG. 16A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga bent surface of a bent housing 9000. In addition, the display portion9001 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, when anicon displayed on the display portion 9001 is touched, an applicationcan be started.

FIG. 16B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal can be used as asmartphone. Note that the speaker 9003, the connection terminal 9006,the sensor 9007, and the like, which are not illustrated in FIG. 16B,can be positioned in the portable information terminal 9101 as in theportable information terminal 9100 illustrated in FIG. 16A. The portableinformation terminal 9101 can display characters and image informationon its plurality of surfaces. For example, three operation buttons 9050(also referred to as operation icons, or simply, icons) can be displayedon one surface of the display portion 9001. Furthermore, information9051 indicated by dashed rectangles can be displayed on another surfaceof the display portion 9001. Examples of the information 9051 includedisplay indicating reception of an incoming email, social networkingservice (SNS) message, call, and the like; the title and sender of anemail and SNS message; the date; the time; remaining battery; and thereception strength of an antenna. Instead of the information 9051, theoperation buttons 9050 or the like may be displayed on the positionwhere the information 9051 is displayed.

FIG. 16C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the displaywithout taking out the portable information terminal 9102 from thepocket and decide whether to answer the call.

FIG. 16D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal 9200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. The portable information terminal 9200 includes the connectionterminal 9006, and data can be directly transmitted to and received fromanother information terminal via a connector. Power charging through theconnection terminal 9006 is possible. Note that the charging operationmay be performed by wireless power feeding without using the connectionterminal 9006.

FIGS. 16E, 16F, and 16G are perspective views of a foldable portableinformation terminal 9201. FIG. 16E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 16F is aperspective view illustrating the portable information terminal 9201that is being opened or being folded. FIG. 16G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thelight-emitting element of one embodiment of the present invention canalso be used for an electronic device which does not have a displayportion. The structure in which the display portion of the electronicdevice described in this embodiment is flexible and display can beperformed on the bent display surface or the structure in which thedisplay portion of the electronic device is foldable is described as anexample; however, the structure is not limited thereto and a structurein which the display portion of the electronic device is not flexibleand display is performed on a plane portion may be employed.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 10

In this embodiment, examples of lighting devices in which thelight-emitting element of one embodiment of the present invention isused are described with reference to FIG. 17 .

FIG. 17 illustrates an example in which the light-emitting element isused for an indoor lighting device 8501. Since the light-emittingelement can have a larger area, a lighting device having a large areacan also be formed. In addition, a lighting device 8502 in which alight-emitting region has a curved surface can also be formed with theuse of a housing with a curved surface. A light-emitting elementdescribed in this embodiment is in the form of a thin film, which allowsthe housing to be designed more freely. Therefore, the lighting devicecan be elaborately designed in a variety of ways. Furthermore, a wall ofthe room may be provided with a large-sized lighting device 8503. Touchsensors may be provided in the lighting devices 8501, 8502, and 8503 tocontrol the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side ofa table, a lighting device 8504 which has a function as a table can beobtained. When the light-emitting element is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

In this manner, a variety of lighting devices to which thelight-emitting element is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example 1

In this example, a method of synthesizing an example of thebenzo[a]anthracene compound which is described in Embodiment 2 andrepresented by the general formula (G1), i.e.,9-[4-(7-benzo[a]anthracene)phenyl]-9H-carbazole (abbreviation: 7CzPaBA,structure formula (100)), is described in detail.

Synthesis of 7CzPaBA

In a 200-mL three-neck flask were put 3.0 g (9.7 mmol) of7-bromobenzo[a]anthracene, 4.4 g (15.4 mmol) of4-(9H-carbazol-9-yl)phenylboronic acid, and 1.1 g (10.5 mmol) of sodiumcarbonate, and the air in the flask was replaced with nitrogen. To thismixture were added 35.0 mL of toluene, 12.5 mL of ethanol, and 9.7 mL ofwater, and the mixture was stirred to be degassed while the pressure inthe flask was reduced. After that, 0.1 g (0.1 mmol) oftetrakis(triphenylphosphine)palladium was added, and the mixture wasstirred at 90° C. for 3.0 hours. Then, this mixture was suctionfiltered, and a residue and a filtrate were separated. The obtainedresidue was dissolved in toluene, and the mixture was suction filteredthrough Florisil, Celite, and alumina to give a filtrate. The obtainedfiltrate was concentrated to give a solid. The obtained solid was washedwith toluene, so that 1.8 g of the object of the synthesis was obtainedin a yield of 39%. This reaction scheme is shown below.

Using a train sublimation method, 1.8 g of the obtained solid waspurified by sublimation. In the purification, the solid was heated at240° C. under a pressure of 3.2 Pa with a flow rate of argon gas of 5.0mL/min. After the sublimation purification, 1.7 g of the object of thesynthesis solid was obtained at a collection rate of 94%.

Nuclear magnetic resonance (¹H NMR) spectroscopy identified thiscompound as 7CzPaBA, which was the object of the synthesis. The ¹H NMRdata of the obtained substance is given below. In addition, ¹H NMRcharts of the obtained substance are shown in FIGS. 18A and 18B.

¹H NMR (CDCl₃, 300 MHz). δ=7.36 (ddd, J=7.7, 7.7, 0.9 Hz, 2H), 7.49-7.88(m, 16H), 8.20-8.24 (m, 3H), 8.94 (d, J=7.8 Hz, 1H), 9.33 (s, 1H).

Characteristics of 7CzPaBA

Thermogravimetry-differential thermal analysis (TG-DTA) of obtained7CzPaBA was performed. A high vacuum differential type differentialthermal balance (TG/DTA 2410SA, manufactured by Bruker AXS K.K.) wasused for the measurement. The measurement was carried out under anitrogen stream (a flow rate of 200 mL/min) and a normal pressure at atemperature rising rate of 10° C./min. From the relationship betweenweight and temperature (thermogravimetry), it was understood that the 5%weight loss temperature was 369° C., which is indicative of high heatresistance.

Absorption and emission spectra of 7CzPaBA in a toluene solution of7CzPaBA are shown in FIGS. 19A and 19B, and absorption and emissionspectra of a thin film of 7CzPaBA are shown in FIGS. 20A and 20B.

The absorption spectra were measured using an ultraviolet-visiblespectrophotometer (V-550 type manufactured by JASCO Corporation). Theabsorption spectrum of 7CzPaBA in the toluene solution was obtained bysubtraction of absorption spectra of toluene and a quartz cell fromabsorption spectra of the toluene solution of 7CzPaBA put in the quartzcell, and is shown in the figure. The absorption spectrum of the thinfilm was obtained by subtraction of an absorption spectrum of quartzfrom absorption spectra of a sample formed by evaporation of 7CzPaBA ona quartz substrate, and is shown in the figure. The emission spectrawere measured with a PL-EL measurement apparatus (manufactured byHamamatsu Photonics K.K.). The emission spectrum of 7CzPaBA in thetoluene solution was measured with the toluene solution of 7CzPaBA putin a quartz cell. The emission spectrum of the thin film was measuredwith a sample formed by evaporation of 7CzPaBA on a quartz substrate.The thin film whose absorption and emission spectra were measured wasformed over a quartz substrate by a vacuum evaporation method. Thethickness of the thin film was 50 nm.

The maximum absorption wavelengths of 7CzPaBA in the toluene solutionwere around 392 nm, around 370 nm, around 355 nm, and around 341 nm. Themaximum emission wavelengths thereof were around 396 nm and around 418nm (an excitation wavelength of 342 nm). Furthermore, the maximumabsorption wavelengths of the thin film were around 395 nm, around 376nm, around 360 nm, around 345 nm, around 331 nm, and around 301 nm. Thelongest maximum emission wavelength thereof was around 429 nm (anexcitation wavelength of 378 nm).

The ionization potential of the thin film of 7CzPaBA was measured in theair with a photoelectron spectrometer (AC-3, produced by Riken Keiki,Co., Ltd.). The obtained value of the ionization potential was convertedinto a negative value, and the HOMO level of 7CzPaBA was −6.02 eV. Fromthe data of the absorption spectrum of the thin film in FIG. 20A, theabsorption edge of 7CzPaBA-02, which was obtained from Tauc plot with anassumption of direct transition, was 2.98 eV Thus, the optical energygap of 7CzPaBA in the solid state was estimated at 2.98 eV; from thevalues of the HOMO level obtained above and this energy gap, the lowestunoccupied molecular orbital level (also referred to as LUMO level) of7CzPaBA can be estimated at −3.04 eV. This reveals that 7CzPaBA in thesolid state has an energy gap as wide as 2.98 eV.

Example 2

In this example, a light-emitting element of one embodiment of thepresent invention in which a delayed fluorescence component due to TTAaccounts for a large proportion of emissive components is described indetail using FIGS. 21A and 21B, FIG. 22 , FIG. 23 , and FIG. 24 .

In this example, light-emitting elements (a light-emitting element 1 anda light-emitting element 2) each corresponding to the light-emittingelement 150 in FIG. 1A and comparative light-emitting elements (acomparative light-emitting element 1 and a comparative light-emittingelement 2) were fabricated, and the fluorescence lifetimes andcharacteristics of the light-emitting elements were measured.

The structures and abbreviations of the compounds used are given below.

<Fabrication of Light-Emitting Elements> <<Fabrication of Light-EmittingElement 1>>

As the electrode 101, a film of indium tin oxide containing siliconoxide (abbreviation: ITSO) was formed to a thickness of 110 nm over asubstrate. Note that the area of the electrode 101 was 4 mm² (2 mm×2mm).

Next, the EL layer 100 in which a plurality of layers were stacked wasformed over the electrode 101. As the hole-injection layer 111,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)and molybdenum oxide (MoO₃) were deposited by co-evaporation to athickness of 60 nm such that the weight ratio of PCPPn to MoO₃ was1:0.5. Note that co-evaporation is an evaporation method in which aplurality of different substances are concurrently vaporized fromrespective different evaporation sources. As the hole-transport layer112, PCPPn was deposited by evaporation to a thickness of 30 nm.

Next, as the light-emitting layer 130,9-[4-(7-benzo[a]anthracene)phenyl]-9H-carbazole (abbreviation: 7CzPaBA)was deposited to a thickness of 25 nm.

Over the light-emitting layer 130,2,2′-(pyridine-2,6-diyl)bis(4,6-diphenylpyrimidine) (abbreviation:2,6(P2Pm)2Py) was deposited to a thickness of 25 nm as theelectron-transport layer 118. Next, as the electron-injection layer 119,lithium fluoride (LiF) was deposited by evaporation to a thickness of 1nm.

As the electrode 102, aluminum (Al) was deposited to a thickness of 200nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 1 was sealed by fixing a sealing substrate to thesubstrate provided with the EL layer 100 using a sealant for an organicEL device. Specifically, a sealant was applied to surround the EL layer100 formed over the substrate, the substrate was bonded to the sealingsubstrate, and irradiation with ultraviolet light having a wavelength of365 nm at 6 J/cm² and heat treatment at 80° C. for one hour wereperformed. Through the above steps, the light-emitting element 1 wasobtained.

<<Fabrication of Light-Emitting Element 2 and Comparative Light-EmittingElements 1 and 2>>

The light-emitting element 2 and the comparative light-emitting elements1 and 2 were fabricated through the same steps as those for theabove-mentioned light-emitting element 1 except for the step of formingthe light-emitting layer 130.

As the light-emitting layer 130 of the light-emitting element 2, 7CzPaBAandN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) were deposited by co-evaporation toa thickness of 25 nm such that the weight ratio of 7CzPaBA to1,6mMemFLPAPrn was 1:0.03. In the light-emitting layer 130, 7CzPaBAserves as a host material and 1,6mMemFLPAPrn serves as a guest material(fluorescent material).

As the light-emitting layer 130 of the comparative light-emittingelement 1,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) was deposited to a thickness of 25 nm.

As the light-emitting layer 130 of the comparative light-emittingelement 2, cgDBCzPA and 1,6mMemFLPAPrn were deposited by co-evaporationto a thickness of 25 nm such that the weight ratio of cgDBCzPA to1,6mMemFLPAPrn was 1:0.03. In the light-emitting layer 130, cgDBCzPAserves as a host material and 1,6mMemFLPAPrn serves as a guest material(fluorescent material).

Element structures of the fabricated light-emitting elements (thelight-emitting elements 1 and 2 and the comparative light-emittingelements 1 and 2) are shown in detail in Table 2.

TABLE 2 Reference Thickness Weight Layer numeral (nm) Material ratioLight-emitting Electrode 102 200 Al — element 1 Electron-injection 119 1LiF — layer Electron-transport 118 25 2,6(P2Pm)2Py — layerLight-emitting 130 25 7CzPaBA — layer Hole-transport 112 30 PCPPn —layer Hole-injection 111 60 PCPPn:MoO₃ 1:0.5 layer Electrode 101 110ITSO — Light-emitting Electrode 102 200 Al — element 2Electron-injection 119 1 LiF — layer Electron-transport 118 252,6(P2Pm)2Py — layer Light-emitting 130 25 7CzPaBA: 1:0.03 layer1,6mMemFLPAPrn Hole-transport 112 30 PCPPn — layer Hole-injection 111 60PCPPn:MoO₃ 1:0.5 layer Electrode 101 110 ITSO — Comparative Electrode102 200 Al — light-emitting Electron-injection 119 1 LiF — element 1layer Electron-transport 118 25 2,6(P2Pm)2Py — layer Light-emitting 13025 cgDBCzPA — layer Hole-transport 112 30 PCPPn — layer Hole-injection111 60 PCPPn:MoO₃ 1:0.5 layer Electrode 101 110 ITSO — ComparativeElectrode 102 200 Al — light-emitting Electron-injection 119 1 LiF —element 2 layer Electron-transport 118 25 2,6(P2Pm)2Py — layerLight-emitting 130 25 cgDBCzPA: layer 1,6mMemFLPAPrn 1:0.03Hole-transport 112 30 PCPPn — layer Hole-injection 111 60 PCPPn:MoO₃1:0.5 layer Electrode 101 110 ITSO —

<Measurements of Fluorescence Lifetimes>

The fluorescence lifetimes of the light-emitting elements 1 and 2 andthe comparative light-emitting elements 1 and 2 were measured. For thelight-emitting element 1, blue light emitted from 7CzPaBA was observed.For the comparative light-emitting element 1, blue light emitted fromcgDBCzPA was observed. For the light-emitting element 2 and thecomparative light-emitting element 2, blue light emitted from1,6mMemFLPAPrn, which is a fluorescent material, was observed. Apicosecond fluorescence lifetime measurement system (manufactured byHamamatsu Photonics K.K.) was used for the measurements. To measure thelifetimes of fluorescence in the light-emitting elements, a square wavepulse voltage was applied to the light-emitting elements, andtime-resolved measurements of light, which was attenuated from thefalling of the voltage, was performed using a streak camera. The pulsevoltage was applied at a frequency of 10 Hz. By integrating dataobtained by repeated measurements, data with a high S/N ratio wasobtained. The measurements were performed at room temperature (300 K). Apulse voltage of 5.0 V (for the light-emitting elements 1 and 2) and apulse voltage of 3.5 V (for the comparative light-emitting elements 1and 2) were applied while being adjusted so that currents flowing in thelight-emitting elements have similar values. The measurements wereperformed under the conditions of a pulse time width of 100 μsec, anegative bias voltage of −5V, and a measurement time of 50 μs. Themeasurement results of the fluorescence lifetimes of the light-emittingelements are shown in FIGS. 21A and 21B. In FIGS. 21A and 21B, thevertical axis represents emission intensity normalized to that in astate where carriers are steadily injected (when the pulse voltage isON), and the horizontal axis represents time elapsed after the fallingof the pulse voltage.

The attenuation curves shown in FIGS. 21A and 21B were fitted with anexponential. As a result of the fitting, the fluorescence lifetimes τ ofthe light-emitting elements 1 and 2 were estimated at 2.1 μs and 2.2 μs,respectively, and the fluorescence lifetimes τ of the comparativelight-emitting elements 1 and 2 were estimated at 3.2 μs and 3.1 μs,respectively. Since the lifetime of fluorescence is generally severalnanoseconds, light observed from each of the light-emitting elements 1and 2 and the comparative light-emitting elements 1 and 2 is probablyfluorescence including a delayed fluorescence component.

In the fluorescence measurements described with reference to FIGS. 21Aand 21B, possible causes of the delayed fluorescence other than theformation of a singlet exciton due to triplet-triplet annihilation (TTA)include the formation of a singlet exciton due to recombination ofcarriers remaining in the light-emitting elements when the pulse voltageis OFF. In these measurements, however, since a negative bias voltage−(-5 V) was applied, recombination of the remaining carriers wassuppressed. Therefore, the delayed fluorescence components shown in themeasurement results in FIGS. 21A and 21B were attributed to lightemission due to triplet-triplet annihilation (TTA).

Next, the proportion of the delayed fluorescence component in the totalemissive components was calculated. The proportion of the delayedfluorescence component in each light-emitting element is shown in Table3.

TABLE 3 Proportion of delayed fluorescence in emissive componentsLight-emitting element 1 25% Light-emitting element 2 20% Comparativelight-emitting 15% element 1 Comparative light-emitting 12% element 2

The results show that the proportion of the delayed fluorescencecomponent in the light-emitting element 1 is 25% and higher than that inthe comparative light-emitting element 1. This reveals that TTA occursmore in the light-emitting element 1 including 7CzPaBA, which has abenzo[a]anthracene skeleton, in the light-emitting layer than in thecomparative light-emitting element 1 including cgDBCzPA, which has ananthracene skeleton, in the light-emitting layer.

The results show that the proportion of the delayed fluorescencecomponent in the light-emitting element 2 is 20% and higher than that inthe comparative light-emitting element 2. This reveals that, even whenlight emitted from 1,6mMemFLPAPrn as the guest material (fluorescencedopant) is observed in both elements, TTA occurs more in thelight-emitting element 2 including 7CzPaBA, which has abenzo[a]anthracene skeleton, as the host material than in thecomparative light-emitting element 2 including cgDBCzPA, which has ananthracene skeleton, as the host material.

<Emission Characteristics Light-Emitting Elements>

Then, emission characteristics of the fabricated light-emitting elements1 and 2 and comparative light-emitting elements 1 and 2 were measured.Note that the measurements were performed at room temperature (in anatmosphere kept at 25° C.).

The emission characteristics of the light-emitting elements at aluminance around 1000 cd/m² are shown below in Table 4. FIG. 22 showscurrent efficiency-luminance characteristics of the light-emittingelements, FIG. 23 external quantum efficiency-luminance characteristicsthereof, and FIG. 24 luminance-voltage characteristics thereof.

TABLE 4 Electric External current CIE Current quantum Voltage densitychromaticity Luminance efficiency efficiency (v) (mA/cm²) (x, y) (cd/m²)(cd/A) (%) Light-emitting 4.4 25.0 (0.17, 0.11) 1090 4.3 4.7 element 1Light-emitting 4.0 7.0 (0.15, 0.19) 861 12.4 9.4 element 2 Comparative3.4 30.6 (0.15, 0.05) 900 2.9 5.4 light-emitting element 1 Comparative3.1 9.6 (0.14, 0.16) 939 9.8 8.5 light-emitting element 2

Emission spectrum peaks of the light-emitting element 1 and thecomparative light-emitting element 1 are at 435 nm and 440 nm,respectively. Blue light emitted from 7CzPaBA and cgDBCzPA was observedin the light-emitting element 1 and the comparative light-emittingelement 1, respectively. Emission spectrum peaks of the light-emittingelement 2 and the comparative light-emitting element 2 are at 466 nm and464 nm, respectively. Blue light emitted from 1,6mMemFLPAPrn as afluorescent material was observed in the light-emitting element 2 andthe comparative light-emitting element 2. Thus, each light-emittingelement emits blue light with an emission spectrum peak at a wavelengthgreater than or equal to 400 nm and less than or equal to 550 nm. Sinceonly light emitted from the fluorescent material is observed in thelight-emitting element 2 and the comparative light-emitting element 2,singlet excited energy generated by TTA is transferred from the hostmaterial to the fluorescent material.

The results in FIG. 22 , FIG. 23 , and Table 4 show that thelight-emitting element 1 has higher efficiency than the comparativelight-emitting element 1 and that the light-emitting element 2 hashigher efficiency than the comparative light-emitting element 2. Theresults indicate that the light-emitting elements 1 and 2 each including7CzPaBA in the light-emitting layer have higher emission efficiency thanthe comparative light-emitting elements 1 and 2 including cgDBCzPA inthe light-emitting layer. The blue light-emitting elements with highemission efficiency were thus achieved using, in the light-emittinglayer, 7CzPaBA which has a benzo[a]anthracene skeleton and in which adelayed fluorescence component due to TTA accounts for 20% or more.

The above-described structure can provide a light-emitting element inwhich a delayed fluorescence component due to TTA accounts for 20% ormore of emissive components and which has an emission spectrum peak inthe blue wavelength range. The above-described structure can provide alight-emitting element in which a delayed fluorescence component due toTTA accounts for 20% or more of emissive components and which has atleast one emission spectrum peak at a wavelength greater than or equalto 400 nm and less than or equal to 550 nm.

<Measurements of Singlet Excited Energy Levels and Triplet ExcitedEnergy Levels>

A factor of the delayed fluorescence in the fluorescence measurementsdescribed with reference to FIGS. 21A and 21B may be thermally activateddelayed fluorescence due to reverse intersystem crossing from thetriplet excited state to the singlet excited state. To efficiently causethe reverse intersystem crossing, an energy difference between the S₁level and the T₁ level is preferably 0.2 eV or less. For this reason, toconfirm whether the delayed fluorescence revealed from FIGS. 21A and 21Bis due to TTA, the S₁ levels and T₁ levels of the materials of thelight-emitting layer in each of the above light-emitting elements weremeasured.

The S₁ levels and T₁ levels of 7CzPaBA, cgDBCzPA, and 1,6mMemFLPAPrnwere measured. Note that the light-emitting elements of one embodimentof the present invention are fluorescent light-emitting elements. In thecase of a fluorescent organic material, since intersystem crossing isnot likely to occur and light emitted from the T₁ level is faint,measurement of the T₁ level might be difficult. Therefore, the T₁ levelswere also examined by quantum chemical calculations.

First, to estimate each S₁ level, a thin film (having a thickness ofapproximately 50 nm) was formed over a quartz substrate by a vacuumevaporation method as a thin film sample, and an absorption spectrum ofthe thin film sample was measured. The absorption spectrum was measuredwith an ultraviolet-visible spectrophotometer (V-550 manufactured byJASCO Corporation). Then, the absorption spectrum of quartz wassubtracted from the measured spectrum of the sample. The S₁ level wasestimated from the Tauc plot assuming a direct transition, which wasconstructed from the data of the absorption spectrum of the thin film.

Next, to estimate each T₁ level, phosphorescence measurement wasperformed. The substance used in the light-emitting element which is oneembodiment of the present invention has very high fluorescence quantumyield, and accordingly, phosphorescence from a thin film sampleincluding the material alone is very difficult to directly observe by alow-temperature PL method. Hence, the following method using a tripletsensitizer was employed to measure phosphorescence and estimate the T₁level.

A co-evaporated film in whichtris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃)was added as a triplet sensitizer to the material whose T₁ level was tobe measured was formed. The film was subjected to a low-temperature PLmethod, and the T₁ level thereof was estimated from the measuredphosphorescence spectrum. The measurement was performed using a PLmicroscope, LabRAM HR-PL, produced by HORIBA, Ltd., a He—Cd laser (325nm) as excitation light, and a CCD detector at a measurement temperatureof 10 K. The co-evaporation with Ir(ppy)₃ increases the probability ofoccurrence of intersystem crossing in the fluorescent material to bemeasured. Thus, phosphorescence from the fluorescent material can bemeasured, which is difficult to achieve when the co-evaporation is notemployed.

For the measurement, each thin film was formed over a quartz substrateto a thickness of 50 nm and another quartz substrate was attached to thedeposition surface in a nitrogen atmosphere.

Next, to verify the T₁ levels measured in the above method, quantumchemical calculations of the T₁ levels were performed.

The calculating method is as follows. Note that Gaussian 09 was used asthe quantum chemistry computational program. A high performance computer(ICE X, manufactured by SGI Japan, Ltd) was used for the calculation.

The most stable structure in the lowest excited triplet state and thesinglet ground state was calculated using the density functional theory.As a basis function, 6-311G (d,p) was used. As a functional, B3LYP wasused. Then, the energy of the T₁ levels was calculated from an energydifference between the most stable structures in the singlet groundstate and in the lowest excited triplet state.

The measurement results (actual values) and calculation results of theestimated S₁ levels and T₁ levels are shown in Table 5.

TABLE 5 Singlet excited Triplet excited Triplet excited energy levelenergy level energy level Abbreviation (actual value) (actual value)(calculated value) 7CzPaBA 2.98 eV 1.97 eV 1.92 eV cgDBCzPA 2.95 eV 1.72eV 1.65 eV 1,6mMemFLPAPrn 2.68 eV 1.84 eV 1.73 eV

The above results indicate a small difference between the values of theT₁ levels measured in the above method and those obtained by the quantumchemical calculations. Therefore, the values of the T₁ levels obtainedin this example are sufficiently reliable.

Table 5 also shows that the S₁ level of 7CzPaBA, which has abenzo[a]anthracene skeleton, is an energy level high enough to be usedas the host material 131 when 1,6mMemFLPAPrn, which is a fluorescentmaterial that emits blue light, is used.

Table 5 also shows that an energy difference between the S₁ level and T₁level of each of 7CzPaBA and cgDBCzPA is 0.5 eV or more. If delayedfluorescence is caused by thermally activated delayed fluorescence dueto reverse intersystem crossing from the triplet excited state to thesinglet excited state, an energy difference between the S₁ level and theT₁ level is preferably 0.2 eV or less to efficiently cause the reverseintersystem crossing. Hence, the delayed fluorescence component of thematerial of the light-emitting layer in each of the above light-emittingelements fabricated in this example is attributed not to the thermallyactivated delayed fluorescence but to TTA.

A spectrum peak of fluorescence emitted from the thin film of 7CzPaBAwas at 429 nm (2.89 eV) and a spectrum peak of fluorescence emitted fromthe thin film of cgDBCzPA was at 442 nm (2.81 eV). Accordingly, adifference in equivalent energy value between the peak wavelengths ofthe fluorescence and phosphorescence spectra of 7CzPaBA and cgDBCzPA was0.5 eV or more. This also shows that the delayed fluorescence componentof the material of the light-emitting layer in each of the abovelight-emitting elements fabricated in this example is attributed not tothe thermally activated delayed fluorescence but to TTA. Note that thefluorescence spectra were measured with a PL-EL measurement apparatus(manufactured by Hamamatsu Photonics K.K.).

As described above, a light-emitting element with high emissionefficiency and an emission spectrum peak in the blue wavelength range,in which a delayed fluorescence component due to TTA accounts for 20% ormore, can be fabricated.

This application is based on Japanese Patent Application serial no.2014-222441 filed with Japan Patent Office on Oct. 31, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. Alight-emitting element comprising: a pair ofelectrodes; and an EL layer between the pair of electrodes, wherein adelayed fluorescence component due to triplet-triplet annihilationaccounts for 20% or more of light emitted from the EL layer, and thelight has at least one emission spectrum peak in a blue wavelengthrange.